32 



EMBRYOLOGY 



ECTODERM 



CORTEX 



ENDOPLASM 



ENDODERM 



LOCALIZATION 



MESENCHYME 

 CORTEX 



Fig. 13. Hypothetical lo- 

 calization of the factors for 

 ectoderm, endoderm, and 

 mesoderm (mesenchyme) in 

 the cortex of the sea urchin 

 egg. "Ectoderm" stands for 

 substances or factors which in 

 later development are respon- 

 sible for the formation of the 

 ectoderm of the embryo. Sim- 

 ilarly with "endoderm" and 

 "mesenchyme." 



Where in the egg itself — the unfertilized egg- — might this organization 

 be? It cannot be in the endoplasm, for this region is full of granules and 

 these granules can be shifted. As we have seen, this endoplasm can be dis- 

 turbed by the centrifuging process and the egg still develops normally. The 

 only other possible location of organization is in the outer cortex. This has 

 a much higher consistency or viscosity; it is a relatively stiff substance. In the 

 process of centrifuging, then, it would be this thin outer layer which would 

 remain undisturbed. Therefore it may contain the organization of the egg into 

 the substances which give rise to the three germ layers (Fig. 13). 



Thus we picture the outer layer of the animal hemisphere as containing 

 the factors for ectoderm. The ectodermal factors extend a little below the 

 equator, forming a sort of shell. Next there is a thin shell of protoplasm 

 which contains the factors for the formation of endoderm. And finally at 

 the vegetal pole itself there is a layer of protoplasm in the cortex which 

 causes mesoderm to form. 



The labile nature of organization 



Now, how stable is the organization into the protoplasm of the three 

 germ layers? We pointed out that if the egg is cut in two through the equator 

 the two halves can be fertilized and will develop into deficient larvae (Fig. 



