IV. i INITIAL STRUCTURE OF THE GERM 



243 



substances in the unsegmented egg-. Should they be already 

 segregated (wholly or in part) in distinct regions, as in Ascidians, 

 Molluscs, and Ctenophors, then even the first blastomeres will to 

 that extent be unable, when isolated, to give rise to more than 

 they would have done in the uninjured egg; should they be 

 uniformly distributed about the axis, but unequally along the 

 axis (Nemertines, Echinoderms, Amphioxus), then a limitation of 

 potentialities will appear when the third division separates the 

 animal from the vegetative cells 1 ; should the arrangement be 



v. 



FIG. 148. Diagrammatic sections through the unsegmented egg of 

 Loligo pealii. (From Korschelt and Heider, after Watase.) A is in the 

 transverse, B in the sagittal plane of the future embryo. In B the 

 anterior side (ro.) is more convex than the posterior (h.). d, dorsal (animal 

 pole); v., ventral (vegetative pole) ; I., left ; r., right. The protoplasm is 

 black, the yolk shaded. 



uniform about the egg-centre, then animal and vegetative 

 blastomeres will be alike totipotential (Coelenterata). 



At the same time it would appear to be true that the capacity 

 of total development may be lost for mere lack of material, for 

 Driesch found that while all four (or all eight in the next stage) 

 animal cells of the sea-urchin egg might give rise to a Pluteus 

 larva, the isolated cells would not. The isolated cells may, how- 



1 We may notice that 0. Maas (Arch. Ent. Mech. xxii, 1906) has found 

 that the isolated anterior flagellated and posterior gianular cells of the 

 Amphiblastula larva of Sycon are not equipotential. The latter can fix 

 and give rise to a young sponge, the former cannot. 



R 2 



