THE GERM-PLASM THEORY 409 



to one daughter-nucleus, while the other, that of the macromere, did 

 not receive this kind of determinant. Here then we have an example 

 of dissimilar or differential division. Those who oppose this theory of 

 qualitative division will hardly be likely to admit this, but will rather 

 seek to maintain that ' external influences,' such as relative position, 

 determine which cells are to give rise to the ciliated ribs and which 

 are not. But the fact that artificial displacement of the micromeres 

 leads to a disarrangement of the ciliated comb-plates, of which the 

 ribs are made up, invalidates this suggestion, and at the same time 

 overthrows the interpretation that it may be the cells which lie on 

 particular meridians that are determined by this position to the 

 production of ciliated plates. Obviously, the converse of this is 

 true ; those cells which contain the rib-determinants come to lie in the 

 regular course of development in these eight meridians, and the cells 

 lying between them, though of the same descent (from micromeres), 

 contain no such determinants and therefore form no ribs. But if those 

 cells which are equipped with rib-determinants be artificially dis- 

 placed, then they give rise to swimming-plates elsewhere than on the 

 aforesaid meridians. 



The experiments made by Crampton on a marine Gastropod, 

 Ilyanassa, likewise go to prove that a disintegration or segregation of 

 the primary constituents does occur in the course of development. In 

 this case, when the first two or first four segmentation-cells were 

 artificially separated from each other, they developed exactly as if 

 they still belonged to the complete ovum, that is, each isolated 

 segmentation- cell yielded, respectively, a half or a quarter-embryo. 

 And these ' partial embryos ' were not able in this case to give rise 

 subsequently to the missing parts or to form complete embryos. 



There are thus two contrasted groups of animals, in one of which 

 a segregation of the mass of primary constituents apparently takes 

 place at the very beginning, while in the other it does not take place 

 in the first stages of development, but apparently occurs later on. 

 We may distinguish these two groups, with Heider, as those having 

 ' regulation ova ' and those having ' mosaic ova.' But I do not see 

 that this affords any reason why we should give up our conception of 

 the successive segregation of the germ-plasm into its determinants, 

 even although, as I said before, I may modify it so far as to say that 

 the segregation does not necessarily take place in all groups and 

 species of animals at the same time, but occurs earlier in some and 

 later in others. 



Now that I have shown how the germ-plasm theory may be 

 brought into harmony with the phenomena of ontogeny, I wish to go 



