CLEAVAGE AND DIFFERENTIATION 89 



§2 



It is clear from these experiments that whatever the first mani- 

 festation of differentiation in the embryo may be, it is not to be 

 found in the division of the nuclei of the blastomeres during 

 cleavage. Attention must therefore be turned to the cytoplasm, in 

 order to see whether it, too, is equivalent in the different blastomeres. 



Considering first the case of the newt : the fact that a lateral or a 

 dorsal half of an egg at the 2-cell stage, a blastula or an early 

 gastrula will develop, but that a ventral half will not (p. 53), shows 

 that all the regions are not equivalent; and since this non-equiva- 

 lence cannot reside in the nuclei, it must concern the cytoplasm. 

 Actually, the importance of the orientation of the constriction 

 separating the halves in the experiments described above, has been 

 shown to lie solely in the fact that the presence of some of the 

 organiser area (grey crescent, dorsal lip region) is essential if 

 development beyond the blastula stage is to take place. When the 

 constriction coincides with the plane of bilateral symmetry, the 

 halves will be lateral and each will possess a portion of the region 

 of the grey crescent. But if the constriction is at right angles to the 

 plane of bilateral symmetry and separates a dorsal half from a ventral 

 half, the former will contain the w^hole 



of the region of the grey crescent and --««^' 



will develop, while the latter will not -rj 



contain any portion of the region in 

 question and will not develop. 



A ventral half of an embryo (blastula „. ^ 



or early gastrula) can be made to ^, ^ .■ ' c u 



•^ , ° ^ . The lormation or an embryo 



develop if the dorsal lip of the blastO- with neural tube, somites, and 



pore of another embryo is grafted into notochord, out of a ventral half 



: , , 1 • 1-11 (see fig. 20, c) of a Tn^o« embryo, 



lt,l and this proves conclusively that by grafting an organiser into it. 



the inabihtyof a ventral half to develop (From Bautzmann, H., Arch. 

 is due, not to lack of any nuclear ^'^^^^"'^^^'- ^^' ^927.) 

 material or factors, but to lack of a definite portion of cyto- 

 plasm — the organiser (fig. 38). 



In the newt, therefore, there is already a differentiation of the 

 cytoplasm just after fertilisation and before the first cleavage, and 



^ Bautzmann, 1927. 



