GASTRULATION 





the egg-shell. In these the gastrular lip has a distinct wedge shape 

 this being apparently impressed upon it as it pushes its way between 

 the eggshell and the surface of the yolk. Corroborative evidence 

 is a (Inn led by the numerous mitotic figures found throughout the 



\ 



FIG. 21. Sagittal sections illustrating gastrulation in Z/C///V/W/, ,,. 



!/.f, j;;istrul;ir lip. 



thickness of the archenteric roof which indicate that it is undergoing 

 rapid growth. 



It will be gathered, however, from a consideration of Fig. 20 

 that those parts of the margin of the small-celled region of the egg's 

 surface which are not involuted to form a groove, must also advance 

 over the surface of the yolk, for these parts of the margin form at 

 first (Fig. 20, A and B) practically a great circle of the egg, while 

 in subsequent stages (C and D) they form a curve of gradually 

 diminishing radius. 



The method by which the small-celled area extends is shown 



