Shape in many respects appears to be a function of growth in an organic material.
A few examples will make clear this important relation. A limit is set to increase in the size of a blastosphere by the nature of the material of its walls. Its wall is a membrane, composed of one or more layers of cells; that this may preserve its curvature, a definite pressure from within must be maintained, proportioned to the cohesive force of the cells; at the same time the wall of the sphere must be able to withstand the strain and pressure put upon it by external forces. All these, and many other factors less easy to conceive, must be delicately adjusted to one another. If in any direction a definite limit be exceeded, then either the structure will be destroyed by disintegration of the component parts, or a new shape will be assumed. The latter is the event in the case of a living substance capable of reaction. The blastosphere, growing beyond its limits, folds into a cup-shaped organism. Did we know all the influences affecting the wall of the blastosphere, then we would understand the causes by which growth beyond a definite limit must result in invagination. From the occurrence of the gastrula in all the divisions of the animal kingdom, we may conclude that it is a temporary phase, inevitable in the growth of animals.
There may be noticed here a second connection between shape and organic growth, exceedingly simple in its nature, but of fundamental importance in its consequences. It may be stated in this saying: Growth always must be such as to produce the greatest possible extension of surface. The reason of this is simple, depending on the different natures of inorganic material and living organic material.
A crystal in its mother liquor grows by attracting new particles and depositing them upon its outer surface, according to the kind of crystallisation peculiar to the material of which it is composed. These particles, once crystallised, retain their position even when new layers are deposited on their outer surfaces, and remain unchanged, perhaps, like rock crystals, for thousands of years, until changed outer forces loosen the bonds that bind them.
Organised material cannot grow in this fashion; it takes up material from without, not, like the crystal, arranging it on the outer surface, but ingesting it. Protoplasm cannot become fixed in any condition without being destroyed; it exhibits perpetual interchanges with the outer world; unceasing intake and output is a necessary accompaniment of its life. 'The growth of idioplasm,' as Naegeli strikingly says, 'implies a constancy of perpetual change.'
Thus, growing protoplasm can assume only such shapes as allow it to remain in constant touch with the outer world. A cubical or spherical mass of cells could not grow by the formation of new layers of cells on the outside, for these layers would deprive the centrally placed masses of cells of their conditions of existence. Similarly, an extended membrane of cells or an epithelial layer cannot add indefinitely to its thickness, else would the cells furthest removed from the outside be injured in their relations to surrounding things. To satisfy its essential conditions, protoplasm can grow only with a proportionate extension of its external surfaces. This is secured by the cells becoming arranged in threads and membranes, and its result is that the threads by branching, and the membranes by folding, produce structures whose complexity increases with growth.
This conception that the shape of growing organisms is in many respects the necessary consequence of the specific characters with which protoplasm is endowed, explains the great contrast between animals and plants in their general organisation. The contrast is the result of the difference between animal and plant metabolism, and between the ways in which animals and plants obtain their food. Plant cells elaborate protoplasm from the carbonic acid of the air, water, and easily diffusible solutions of salts, obtained from the sea or from the soil. For the chemical work of combining these, they require the active energy of sunlight. We can now see the chief requirements to which the constitution and arrangement of the cells in a multicellular plant must be adapted. Plant cells may become clothed in a thick membrane, as that would prove no hindrance to the passage of gases and easily diffusible salts; but they must be arranged so as to present the greatest possible surface to the surrounding media (_i.e._, to the soil and the water, the air and the sunlight) whence is drawn their supply of matter and force. The cells must turn a broad face to the outside; this they do by becoming arranged in branching rows, or in leaf-shaped flattened organs.
That they may suck up water and salts from the soil, the cells are arranged as a highly branched system of roots, covered with delicate hairs, and penetrating the soil in every direction. To inhale the carbonic acid from the air, and to be subjected to the influence of sunlight, the aerial part of the plant stretches out its branches towards the light, and becomes folded into the flat leaves, the structure of which reveals a suitability for assimilation. Thus the whole architecture of a plant is superficial and visible; internal differentiation into organs and tissues either is wanting, or, compared with animals, is very scanty. It is only in the higher plants that the internal fibro-vascular tissues appear; these serve a double purpose: they act as channels along which the sap passes, so bringing together the different materials absorbed by roots and leaves; and they have the mechanical function of strengthening the stem and branches.
The different mode of nutrition of animals results in a totally different structural plan. Animal cells absorb material that is already organised, and that they may do so their cells are either quite naked, so affording an easy passage for solid particles, or they are clothed only by a thin membrane, through which solutions of slightly diffusible, organic colloids may pass. Therefore, unlike plants, multicellular animals display a compact structure with internal organs adapted to the different conditions which result from the method of nutrition peculiar to animals. A unicellular animal takes organic particles bodily into its protoplasm, and forming around them temporary cavities known as food vacuoles, treats them chemically. The multicellular animal has become shaped so as to enclose a space within its body into which solid organic food-particles are carried and digested, thereafter, in a state of solution, to be shared by the single cells lining the cavity. In this way the animal body does not require so close a relation with the medium surrounding it; its food, the first requirement of an organism, is distributed to it from inside outwards. In its further complication the animal organisation proceeds along the same lines. The system of internal hollows becomes more complicated by the specialisation of secreting surfaces, and by the formation of an alimentary canal, and of a body cavity separate from the alimentary canal.
In plants, it is the external surface that is increased as much as possible. In animals, in obedience to their different requirements, increase takes place in the internal surface. The specialisation of plants displays itself in organs externally visible--in leaves, twigs, flowers, and tendrils. The specialisation of animals is concealed within the body, for the internal surface is the starting-point for the formation of the organs and tissues.
Comparative embryology shows that, however varied the forms and functions of the numerous animal organs may be, the method of their development is remarkably similar. There are required only the slightest variations of a few simple general laws. For these I may refer readers to a series of special investigations (_Studies on the Germ-layer Theory_, Oscar and Richard Hertwig), and to the fourth chapter of my Embryology, 'General Discussion of the Principles of Development.'
In these works and in the foregoing pages I have tried to show that the multiplication of the egg-cell by division is itself a source of increasing complexity and an active principle in the determination of form, since the products of the division unite to form a higher unity. But in another way the multiplication of cells leads to differentiation among the cells arising from the egg. Although each of these resembles the parent egg, from which they arose by doubling division, yet they differ from it in one point: they are no longer a whole, but have become the subordinate parts of a higher unity, that is, of a higher organism. A cell that is no longer a whole, but the part of a whole, has entered upon reciprocal relations with other cells, and in the functions of its life is limited by these others and by the whole. The further this is carried the more the cell falls short of its independence as an elementary organism, and appears only as a part with its functions subordinate and in dependence upon the whole.[18]
Although from the point of view of morphology it has become more and more imperative to regard the cell as the unit of the higher organism, still, from the physiological point of view the higher organisms must be regarded as masses of material acting as wholes, and composed of several grades of structural parts, subordinate in function to the whole, and displaying only a limited division of capacities. And so the cell theory, according to which the cell was exalted unduly as the unit of life, the centre of life, the elementary organism, must take limitation and correction from these wider views. This has already been insisted upon by many physiologists of insight--for instance, by Naegeli (see p. 30), by Sachs, and by Vochting.
'Cell formation,' declares Sachs (_Physiology of Plants_, p. 73), 'is a phenomenon very general, it is true, in organic life, but still only of secondary significance; at all events, it is merely one of the numerous expressions of the formative forces which reside in all matter, in the highest degree, however, in organic substance.' 'Essentially, every plant, however highly organized, is a continuous mass of protoplasm, surrounded externally by a cell wall and penetrated internally by numerous transverse and longitudinal partitions.'
My conception receives strong support from the way in which Vochting set forth the relations of the cell to the whole:
'Is the circumstance that a cell, separated from the organism, is able to survive and build up the whole again a proof of the independent life of the cells while in the organism? I believe it to be only a proof that the life of the organism is always dependent upon the cell, that the life is inherent in the cell, and that the life of a compound organism is merely the resultant of the vital phenomena of its single cells; but by no means that the cell when isolated displays the same functions as while it is a part of the organism. The cell while in the organism and the cell separated from the organism and self-sufficing, are quite different. We must regard the functions of a cell that is part of an organism, disregarding external influences, as determined by the whole organism, and only by the cell itself, in so far as that forms a greater or less part of the whole organism. When not part of an organism, the cell is independent, and entirely determines its own function. Nowhere is it easier than in this case to confuse possibilities with facts, and nowhere is the confusion more fatal. From a morphological point of view, one may confidently regard the cell as an individual; but it must be borne in mind that an abstraction has been made. Physiologically considered, the cell is an individual only when it is isolated from a complex and is independent; of this no abstraction can be made.'
According to the conception I have been explaining, cells merge their independent individuality in that of the whole, and so the force that directs their ultimate development, and that leads to their appropriate elaboration, cannot be within them, cannot reside in special groups of determinants, in the sense of Weismann. It is given by the relations in which the cells come to stand to the whole organism and to the various parts of the organism, and, on the other hand, to surrounding things.
Naturally, such relations differ with the place or position occupied by cells in the whole organism, and in this way there come to be innumerable conditions making for diverging directions of development, for division of labour, and for dissimilar, histological differentiation. The part played by a cell, as Vochting puts it, will depend upon the position it comes to assume in the whole living unit. To use an expression of Driesch's, dissimilar differentiation of cells is a 'function of position.' Such a conception my brother and I, in our _Studies on the Germ-layer Theory_, sought to establish clearly by many examples from the histology of the coelenterates and of higher animals; such a conception for long has been clearly expressed in physiological botany.
The simpler nature of plants in structure and function makes it easy to conduct experimental observations upon this point.
I have already described how either side of the prothallus of a fern may be made to produce male or female organs, according as it is kept in the light or in the dark. Similarly, taking a willow slip, roots may be made to appear at one end by moisture and darkness, while they will not appear on the end kept in the light.
The experiments of botanists and of fruit-growers show that young buds and the rudiments of roots are indifferent structures, the further growth of which depends entirely upon the conditions in which they are placed. 'One and the same bud may grow to a long or short vegetative shoot, to a floral shoot, to a thorn, or may remain undeveloped. The same root rudiment may grow to a main tap-root or may form a secondary lateral root. The conditions that determine the mode in which these structures will develop are quite within the power of the experimenter. We have shown already and could show further, that he is able to determine the mode of growth by cutting, bending, tying in a horizontal position, and so forth: For such reasons, Vochting describes plants as masses of tissue, practically plastic, and which may be moulded at the discretion of the investigator.
'For instance, in the case of _Prunus spinosa_, a branch may be produced in place of a thorn by cutting a growing shoot at the proper height, in spring. The buds below the point where the cut was made turn to shoots like the rest of the plant and complete the interrupted growth, while on an uncut stem they would have grown to thorns. Thus, the rudiment of a thorn has been changed to that of a shoot' (Vochting).
Although it is more difficult to carry out experiments upon animals, some good instances are known. If a piece cut from the stem of _Antennularia_ (a hydroid polyp) be placed vertically, in a short time new branches and new 'roots' spring from it. In this case, again, the position of the new growths is determined by the relation in which the stem is placed to gravity. 'The tentacles arise only at the end turned towards the zenith; the "roots" from the parts directed towards the ground' (Loeb).
A similar example may be taken from among vertebrates. The notochord arises from a set of cells which are in close relation with the fused tips of the blastopore. By exposing developing frog's eggs to abnormal conditions, I was able, in some cases, to produce a hypertrophy of one of the lips of the blastopore. When fusion of the lips took place the normal lip united with the rim of the protruding hypertrophied lip. As a result of this the notochord and the nerve plate came to arise, not from the usual set of cells, but from those cells that, by the abnormal condition, had come to lie in the place for the notochord. The protruding cells, which normally would have developed into notochord and nerve plate, grew into a simple fold of the external skin.
Moreover, it is well known in pathology that mucous membranes may lose their proper character and assume the qualities and aspect of the external skin, when, as in cases of prolapse, fistula, etc., they have been exposed for some time to the air.
The relations of different parts to each other and to the whole are known as correlations. Correlation exists in all the stages of the development of an organism, sometimes in one way, sometimes in another. One must note very carefully that Weismann's doctrine of determinants, according to which all that happens in development follows a prearranged plan, is entirely in opposition to this correlative character of the changes that occur during development.
Here I shall give a few quotations from botanical and zoological writers:
'If the stem of a plant be cut so that it retains its roots, but is deprived of leaves and shoots, then the adventitious buds will produce new leaves and shoots. If, however, the stem be cut so as to deprive it of roots, then the same cells that in the other case produced leaves and shoots will now produce roots. Precisely the same occurs with a piece of the root. In fact, it appears as if the idioplasm knew what parts of the plant were wanting, and what it must do to restore the integrity and vital capacity of the individual.' 'The idioplasm in the remaining part of a plant must be affected when an important part has been removed, because the idioplasm of the lost part is no longer capable of having influence.' 'It is clear enough that necessity acts as a stimulus, and that each definite need calls into existence the appropriate reaction.'
These are Naegeli's views, and they have been elaborated by Pfluger in his important treatise on _The Teleological Mechanism of Living Nature_ (1877).
Vochting writes in similar fashion:
'In a tree that is growing under normal conditions, without being subjected to injury, all the organs appear in definite relation to each other: so many leaves correspond to a definite number of twigs and branches. These spring from a stem of proportionate thickness, and the stem passes into a definitely proportioned tap-root, from which arise a due array of lateral roots. In normal conditions all these organs are in equilibrium. An apple-tree, growing on the line where tilled garden ground meets a lawn, grows more vigorously on the side towards the garden. If one of the roots of an apple-tree with three main roots and three branches be amputated, then the corresponding branch will lag behind in growth, although it may not absolutely perish.' 'The equilibrium varies according to the specific nature of the tree. It is shown in one way in the oak, in another in the beech, and is different in the varieties of a species.'
Finally, consider this statement from Goebel's _Treatise on the Morphology and Physiology of the Leaf_: 'The fact that lateral buds do not develop while the axial bud is still growing vigorously depends upon the relation between the two. That I denote as correlation of growth.'
The dependence of parts upon each other, and upon the whole, is specially clear and instructive in cases where different plant individuals are united by budding or grafting. To limit the growth of a tree, and to induce it to become dwarfed, it is necessary only to graft it upon a nearly allied but dwarf variety. When a pear-tree is grafted upon the quince, which is characterized by its dwarf-like growth, the vegetative growth of the pear is reduced exceedingly. It produces shorter and weaker shoots; all the dwarf varieties of the pear employed as wall fruits, or growing into the little pyramids spoken of in the trade as 'cordon'-trees and potting-trees, could not have been produced unless the gardener had had the quince as a natural dwarf stock (Vochting). With the dwarfing is associated a freer and earlier production of fruit. Other kinds of fruit-trees, apples, apricots, and so forth, show the same course.
'The capacity to withstand external influences and the duration of life may be altered in the same way. The pistachio (_Pistazia vera_), cultivated in Frankfort, which is destroyed by a temperature lower than 7.5 degrees of frost, will survive 12.5 degrees if it has been grafted upon _P.
terebinthus_. Moreover, when it is grown from a seedling, it may reach the age of 150 years; but when it has been grafted upon _P. terebinthus_ its length of life is increased to 200 years; while, grafted on _P. lentiscus_, it reaches only about 40 years' (Vochting).
Vochting's experiments upon beetroot are still more characteristic. 'The stem of a beet plant that bore young buds gave rise to vegetative shoots when it was united with a young, still growing root, but to a blossoming stem when it had been grafted, in spring, upon an old root.'
Similarly, animal growth is correlative in all its stages. When a muscle becomes unusually large it sets up corresponding correlations of growth in many other parts of the body. The bloodvessels and nerves supplying it become larger, and the increase in the nerves leads to corresponding increase in the nerve centres. The tendons of origin and of insertion, and the parts of the skeleton to which these are attached, must react to the increased size of the muscle by growing larger; in fact for all the parts of the animal body the conclusions which Naegeli and other physiologists drew from plants are applicable. All the different elements of the body are in definite and intimate touch with each other.
This is shown most beautifully and clearly in the extraordinarily interesting phenomena called dimorphism and polymorphism. These seem to me to show how very different final results may grow from identical rudiments, if these, in early stages of development, be subjected to different external influences.
Finally, I have a little to say about the sexual dimorphism that occurs so generally in the animal kingdom.
Nearly all kinds of animals appear as male or as females. These differ from each other not only in that they produce eggs or spermatozoa, but frequently in a number of more or less striking characters affecting different parts of the body, and known as secondary sexual characters. In fact, the difference between the sexes may be so great that a systematic naturalist, unacquainted with the mode of development of the creatures, might place them in different species, genera, or even families, on account of the striking differences in external characters.
As an instance, take _Bonellia_, a gephyrean, the strange case of which has been remarked upon by Hensen and by Weismann. The male is about a hundred times smaller than the female, in the respiratory chamber of which it lives as a kind of parasite, and appears, so far as outward shape goes, more like a turbellarian than a gephyrean. None the less, male and female are alike not only while they are in the egg, but as larvae, and it is only towards the period of sexual maturity that the great difference between them begins to appear. So also is it with the dwarf males of the cirripedes.
Males and females, whether they be more or less unlike, arise from the same germinal material. The germinal material itself is sexless; that is to say, there is not a male and a female germinal material. The phenomena of inheritance in the sexual generation of hybrids show this clearly.
Characters appropriate both to males and to females are transmitted either by eggs or by spermatozoa. In parthenogenetic animals both male and female individuals appear at definite times from eggs produced without sexual commerce. Whether the male or the female forms be produced depends, not upon any difference in the germinal material, but on the external influences, just as external influences determine whether the bud on a twig shall give rise to a vegetative or to a flowering shoot, to a thorn or to a stem. The influence of food, of temperature, or probably of other agencies, determines in which direction the germinal material shall grow.
The experiments of a distinguished French investigator, M. Maupas, on the determination of sex in _Hydatina senta_, a rotifer, have given striking results.
In _Hydatina_, under normal conditions the eggs of certain individuals give rise always to males, of others always to females. By raising or lowering the temperature at the time when the eggs are being formed in the germaria of the young females, the experimenter is able to determine whether these eggs shall give rise to males or to females. After that early time the character of the egg cannot be altered by food, light, or temperature.
In one experiment, in which five females not yet fully grown were kept in a room at the temperature of 26 to 28 degrees centigrade, Maupas found that, of 104 eggs only 3 per cent. gave rise to females, while in the case of other five young females of the same brood, but kept in a cold chamber at a temperature of 14 to 15 degrees centigrade, 95 per cent. of females were produced. In another experiment, young animals were kept for a few days in the cold, and then, until death, in a higher temperature. Of the eggs produced while in the cold, 75 per cent. produced females, of those deposited in the warmth, 81 per cent. became males.
With these results may be compared what happens with many plants. Melons and cucumbers, which produce on the same stem both male and female flowers, bear only male flowers in high temperatures, only female flowers when subjected to cold and damp.
In the case of many insects in which parthenogenesis occurs, the determination of sex depends upon fertilisation. Thus, among bees, unfertilised eggs give rise to drones, fertilised eggs to females.
Sexual dimorphism in still another way reveals the intimate interactions existing between all the parts of an organism in every stage of development. It is well known, for instance, that among animals the early removal or destruction of the sexual organs hinders the development of the secondary sexual characters, or even may occasion the appearance of the characters of the other sex. Old hens become cock-feathered; human eunuchs have the high-pitched voice and the peculiarities of the larynx found in women.
As much as sexual dimorphism, the phenomena of polymorphism show the enormous influence exerted by external forces upon correlated variation of the parts during development, and in this way upon the final structure.
In the question of polymorphism it is worth while to discuss at some length the extreme polymorphism exhibited in the case of some of the colonial animals--first, because the matter has recently occasioned an important controversy between Herbert Spencer and Weismann; and, secondly, because the discussion will serve to make still more clear the difference between my views and those of Weismann upon the nature of the process of development.
Among the colonial insects there arise, in addition to males and females, sexless individuals known as neuters. These in certain cases are very different from both males and females in structure and in social instincts.
Among bees there are the queens, sexually mature females; the workers, females whose sexual organs are rudimentary, and parts of whose bodies--the stings, the wings, the hind legs, with their pollen-collecting apparatus--are peculiarly formed; and, lastly, the males, or drones.