On the other hand, in differentiating division the rudiments become irregularly grouped during their growth; consequently, on division of the ids, which are composed of determinants, totally different combinations of the determinants are included in the daughter-ids. This method of division of the germplasm plays the chief part in the transformation of the egg into the adult. It has to take place so that the numberless determinants, or guiding particles, of the germplasm may be disentangled and brought forward at the time and place necessary for them to guide the formations of the determinates, or independently variable parts of the adult body.
To take an example: Weismann's hypothesis requires that when the egg first divides into two, the germplasm should divide into two halves, each containing only one half of the total assemblage of determinants. In each subsequent cell-division this process of segregation is continued, so that the ids, as the phases of embryonic growth occur, contain more and more few different kinds of determinants. Supposing the germplasm to be composed of a million determinants at one stage, in the next it would contain only half a million, and in the next, again, only a quarter-million. In this manner the architecture of the ids becomes simpler and simpler, reaching the simplest conceivable condition in the active cells of the adult body. In these the germplasm consists only of the kind of determinants peculiar to the cells in which they lie; and these determinants are broken up into biophores, or bearers of cell qualities.
'The disintegration of the germplasm,' says Weismann,[7] 'is a wonderfully complicated process; it is a true "development," in which the idic stages necessarily follow one another in a regular order, and thus the thousands and hundreds of thousands of hereditary parts are gradually formed, each in its right place, and each provided with the proper determinants. The construction of the whole body, as well as its differentiation into parts, its segmentation, and the formation of its organs, and even the size of these organs—determined by the number of cells composing them—depend upon this complicated disintegration of the determinants in the id of germplasm. The transmission of characters of the most general kind—that is to say, those which determine the structure of an animal as well as those characterising the class, order, family, and genus to which it belongs—are due exclusively to this process.'
This mechanism of differentiating division fails to explain the phenomena of reproduction and of regeneration. For these Weismann has the following ancillary suppositions:
The first is the already-described hypothesis of continuity of the germplasm. As the disintegration of the germplasm into determinants, occurring in the development of an egg into an organism, is a process which cannot be retraced, and, as the future reproductive cells of the organism must contain undisintegrated, perfect germplasm, it follows that the germplasm in the germ-cells of the child must have come directly from the original germplasm of the parent. During the development, as Weismann assumes, only a few of the ids, each of which contains all the necessary germs, break up by differentiating division into the determinants which control the course of the ontogeny, and decide the final characters of the cells. Another set of ids remains undisintegrated, with their determinants fast bound together, and, in the cell divisions, is not broken up into dissimilar groups. The first set of ids is the active, disintegrating germplasm; the second set is a passive, latent germplasm, which may be described as accessory germplasm (Nebenkeimplasma). The active ids are his explanation of the embryonic events, which they direct; the accessory germplasm is reserved to form the germ-cells, and, in fast-bound condition, is handed on through a short or long series of cell-divisions alongside the active germplasm. Handed on in this passive state, it finally reaches a group of cells which may be many or few generations distant from the original egg-cell, and impresses upon them the character of sexual cells. This transfer of germplasm from the egg to the sexual cell occurs in orderly fashion, along prescribed series of cells which Weismann has called the germ-tracks. Only these cells, which contain part of the perfect, undisintegrated germplasm, serve for the preservation of the species and are immortal; the other cells, since, from the disintegration due to differentiating division, they contain only fragments of the perfect plasm (groups of determinants or single determinants), are mortal, somatic cells.
The formation of buds is explained in much the same way as the origin of germ-cells. There is handed along from the egg, through prescribed series of cells, a quantity of accessory, or bud, idioplasm.
The phenomena of alternation of generations require the supposition that in those animals and plants in which it occurs 'two kinds of germplasm exist, both of which always are present in the egg or in the bud, but of which one only is active at any time and rules the ontogeny, while the other remains inactive.' The alternating activity of these two produces the alternation of generations. So also dimorphism, which is exhibited most frequently as differences between the sexes, is explained by the assumption that 'double determinants' are present in the germplasm for all the cells, cell-groups, or entire organisms which have different characters in the male and female. One set of these double determinants remains latent, the other becomes active.
Finally, to explain the phenomena of regeneration, it is assumed that in the complicated cases where large parts of the body, like the head, the tail, or a bone, can be replaced after accidental loss, the cells with this power of regeneration contain, in addition to the determinants proper to them, supplementary determinants, which contain the germs needed for regeneration of the lost parts. These were handed on, during the ontogeny, through definite series of cells, in a passive condition, to become active when the conditions for their growth are supplied by the loss of the parts they can replace.
Criticism of the Germplasm Theory.[8]
At first sight, much of Weismann's fabric of hypotheses gives the impression of being a closed system, thought out as a whole, and it has been treated as such in most of the notices and criticisms which I have seen. As a matter of fact, Weismann has spared no pains in the elaboration of his system, and has attempted to bring under his theory the many different phenomena of heredity and development, as well as alternation of generations, regeneration, atavism, and so forth. But, on the other hand, he has been careless in testing the stability and security of the foundations upon which he has built. It is on solid foundations that lie deep in the earth, and that avoid all reproach of being scamped or superficial work, that the durability of a structure depends. In this criticism the details of the superstructure will be disregarded, but the foundation will be tested thoroughly.