IV. i INITIAL STRUCTURE OF THE GERM 245 



pore is in very numerous cases formed at the vegetative pole, the 

 bilateral symmetry of the eggs of Cephalopoda (Figs. 148, 149), 

 Insecta, some Crustacea, and Ascidia becomes that of the 

 embryo ; in Ascaris- the first four cells lie in one plane, which 

 becomes the median plane, the germ-cell is posterior and the 

 animal pole dorsal ; and so on. 



(3) The instances in which the segregation of the organ- 

 forming substances into the blastomeres takes place at once are 

 especially interesting, as they appear to fulfil exactly the require- 

 ments of the ( Mosaik-theorie ', if that doctrine be transferred from 

 the nucleus to the cytoplasm, for here, undoubtedly, segmentation 

 is a qualitative process, a sundering of potentialities. It must be 

 pointed out, however, that even in these cases the factors which 

 determine segmentation and those which determine differentia- 

 tion may be as distinct as they are elsewhere, 



In the Frog there is a much greater correlation between the 

 symmetry of the egg and the median plane of the embryo than 

 between the former and the plane of the first furrow. 



A \ or i blastomere of a Nemertine or a sea-urchin segments 

 as a part, and yet it eventually develops into a whole larva : and 

 the same is true of egg-fragments. All the isolated blastomeres 

 of a Mollusc segment as parts, yet in Dentalium, that one which 

 contains all the necessary stuffs, CD or D, can give rise to a 

 complete embryo, while its fellows cannot. The converse case 

 is to be found in Ctenophors; here the egg which has been 

 deprived of its vegetative portion segments as a whole and yet 

 produces an embryo devoid of costae and sense-organs. 



There are also certain forms (Amphioxus and Veiteb rates, 

 Coelenterata) where the isolated cell segments as a whole, and 

 this appears to depend on the rapidity with which the part can re- 

 arrange its material or resume the polarity of the whole (Wetzel). 

 It is further to be noticed that the capacity of a part for total 

 segmentation, like its power of total differentiation, may differ in 

 the same egg at different times and under different conditions ; the 

 fragment of an immature egg of Cerebratulus divides like a whole 

 egg, an isolated blastomere like a part ; again, in Ilyanassa the 

 cleavage of apart may be made total by lowering the temperature. 



When, further, it is remembered how closely similar the 



