66 EMBRYOLOGY 



be understood only by the study of three-dimensional models in the labora- 

 tory or by watching a film of gastrulation of the living egg. The cellular 

 movements will be outlined in more detail in Chapter 9. 



Gastrulation and the formation of 

 the nervous system 



The real significance of this process of gastrulation is twofold. First of 

 all, it brings the presumptive organs into their normal position in the embryo. 

 Internal structures originally are presumptive areas on the external surface 

 of the spherical blastula and invaginate to assume their proper positions. 



The second important aspect of gastrulation is that it is necessary for the 

 formation of the nervous system. This fact can be demonstrated very simply 

 by examining embryos in which normal gastrulation fails to take place. In 

 some amphibian eggs, if the outer protective membranes are removed, the 

 eggs form exogastrulae instead of normal gastrulae. In an exogastrula the 

 early gastrula looks fairly normal. But regions 1 through 5 and A through F. 

 instead of invaginating, evaginate — that is, grow outward — so that the 

 gastrula becomes converted by this abnormal process into an exogastrula 

 (Fig. 29). Regions 1 through 5 extend along the dorsal side. Regions A 

 through F spread out along the ventral region. Thus the mesoderm and 

 endoderm are not covered with ectoderm. The ectoderm remains isolated 

 from the rest of the tissues. 



The important thing about exogastrulation is that although endodermal 

 and mesodermal structures develop to a great extent, no nervous structures 

 form. The notochord, mesodermal somites, gills, and lining of the gut differ- 

 entiate as illustrated — that is, there is a fairly normal differentiation of the 

 mesodermal and endodermal derivatives. However, neither the neural plate 

 nor any of the sensory structures develop from the ectoderm if the regions 

 1 through 5 fail to invaginate and thus do not make contact with the 

 ectoderm which normally forms the neural plate. Therefore the development 

 of all the sensory structures and the nervous system is dependent upon the 

 normal contact between the roof of the archenteron and the ectoderm. And, 

 as can be seen from Figure 28, in normal gastrulation the entire region from 

 1 through 5 normally does invaginate and comes in contact with the pre- 

 sumptive neural plate. Under these conditions normal development of the 

 nervous system is obtained. 



