PRE-GASTRULATION STAGES 103 



continuity of the epibolic movement of the surface coat toward the 

 vegetal hemisphere. This is only an apparent interruption, since it in- 

 volves an inturning of cells at a very specific region of the marginal 

 zone, or the most ventral limit of the pigmented animal hemisphere 

 cells. This specific region is the ventral limit of the original gray cres- 

 cent, destined (at the time of fertilization) to become the region of 

 formation of the dorsal lip of the blastopore, with all of its implica- 

 tions. 



Cells which lie on the lateral surface of the late blastula begin to 

 roll inwardly, first only a few cells and then, by a sort of contagion, the 

 contiguous cells of the more lateral marginal zone. It is this infolding 

 process which marks the actual, observable process of gastrulation. It 

 must be emphasized that these observable changes are probably long 

 anticipated, as will be suggested by the detailed description below. If 

 there is interference with this inturning movement of cells, any sub- 

 sequent development is apt to be abnormal or incomplete if, in fact, 

 it occurs at all. Embryos at the time of gastrulation are indeed hyper- 

 sensitive to physical changes in the environment, and to genetic in- 

 compatibilities within the chromosomes of the involuting cells. Em- 

 bryos which survive this process may reasonably be expected to 

 achieve the next major step, namely neurulation. 



Pre-gastrulation Stages 



There is no clear-cut demarcation between the blastula and the 

 gastrula stages, unless one accepts the initial involution of the dorsal 

 lip cells. Some investigators have pointed out that there is a dispro- 

 portionate ratio of the yolk and cytoplasm to the nucleus of the fer- 

 tilized egg, as compared with the ratio in the somatic cell. This sug- 

 gests to them that cleavage results in a progressive approach to the 

 somatic ratio of nuclear volume to cytoplasmic volume. When the 

 ratio is reached whereby the nucleus can properly control its cyto- 

 plasmic sphere of influence, then the latent influence can begin to exert 

 itself and the process of differentiation begins. Such a situation occurs 

 at the beginning of the gastrula stage. Cleavage continues, of course, 

 but it has been proved beyond a doubt that cell areas are no longer 

 of equivalent potency with regard to ultimate development. It has been 

 suggested, therefore, that the blastula stage is monodermic (one prin- 

 cipal outer layer of cells, generally arranged somewhat spherically) 

 and contains a blastocoel. The end of the blastula stage is reached 



