72 DIFFERENTIATION III 



It is, of course, from the former that the structure in question 

 is derived in the development of the whole egg. 



We shall discuss the significance of this later on. It has 

 been pointed out above that the egg-cytoplasm acquires its 

 definitive bilateral structure as a result of fertilization. It is 

 interesting that the removal of a part of the cytoplasm before 

 the appearance of the grey crescent has far less serious conse- 

 quences than after the bilateral symmetry has been estab- 

 lished. In the first case the embryo is normal, in the second 

 while it may be normal, it is frequently defective in certain 

 respects or altogether unable to develop (Brachet). 



In Amphioxus, Echinoids, and Nemertines the egg has 

 a telolecithal structure and a polar radial symmetry. In 

 Amphioxus the radial is replaced by a bilateral symmetry as 

 a result of fertilization (Cerfontaine) exactly as in the Frog 

 and in Ascidians (Conklin). At what moment the bilaterality 

 is determined in the other two cases is not known. 



In all three groups the first two and again the first four 

 blastomeres have similar shares of the parts of the polar struc- 

 ture, since the first two divisions are meridional, and in all three 

 isolated half- and quarter-blastomeres give rise to whole 

 embryos or larvae of reduced size. But by compressing the 

 egg of Cerebratulus at right angles to its axis the second 

 division may be made equatorial ; the quarter-blastomeres are 

 then either animal or vegetative, and behave when isolated 

 like the one-eighth-blastomeres of the normal egg (Yatsu). 

 The next division, however, separates animal from vegetative 

 one-eighth-blastomeres, and these are no longer totipotent, at 

 least not invariably so. In Amphioxus neither the animal nor 

 the vegetative cells can gastrulate; in the Nemertines the 

 vegetative cells gastrulate but form no sense-organ, the animal 

 cells form a sense-organ but no gut ; and in the Sea-urchin 

 the animal cells usually give rise to long ciliated blastulae, 

 though they may gastrulate, while the vegetative cells de- 

 velop more frequently into gastrulae with a pair of skeletal 

 spicules. 



There is therefore a difference between the cells derived 



