112 GASTRULATION 



endoderm cells toward the gray crescent side become separated 

 from the epiblast and are forced upward, displacing the blas- 

 tocoel and reducing its size. This process is known as pseudo- 

 invagination because there is a degree of "pushing in" of the 

 vegetal hemisphere cells. It is not true invagination, however, as 

 one understands the process in such a form as the starfish 

 embryo. The slit-like space between the yolk endoderm and the 

 epiblast, continuous with the blastocoel, is sometimes referred to 

 as the gastrular slit. 



3. The initial involution or inturning of a jew cells at the lower 

 margin of the original gray crescent, followed by the lateral ex- 

 tension of this involution along the epibolic marginal zone. This 

 region of initial involution becomes the dorsal lip of the blas- 

 topore. The marginal zone cells become separated from the more 

 ventral and lighter colored yolk cells. The first cells to involute 

 do not lose continuity with their neighbors, but carry with them 

 the adjacent, contiguous lateral cells of the marginal zone. The 

 infolding or inturning process therefore begins at one point but 

 is continued around the marginal zone. 



4. Further epiboly of the entire marginal zone toward the vegetal 

 hemisphere, apparently interrupted only at the levels of involu- 

 tion. The inturning cells are rolling under themselves like an on- 

 coming wave. The margin of the wave continues, through 

 epiboly, to move toward the vegetal hemisphere but at a rate 

 which seems slower than that of the marginal zone which has 

 not yet involuted (i.e., opposite the dorsal lip cells). In other 

 words, cells are moving inwardly over the dorsal lip of the blas- 

 topore, but the margin of the lip itself is moving vegetally as a 

 result of epibolic extension. Some epibolic movement is ab- 

 sorbed in involution. 



5. The piling up or confluence of animal hemisphere cells, many 

 of which are destined to move inside over the blastoporal lips. 



6. The continued lateral extension of the involuting marginal zone 

 cells so that there is eventually a circumferential meeting of the 

 blastoporal lips. These comprise the dorsal, lateral, and ventral 

 lips of the blastopore, not clearly demarked. The vegetal hemi- 

 sphere cells, which are now exposed within a ring of involuted 

 marginal zone (lip) cells, are collectively known as the "yolk 

 plug." The term yolk plug is used because the cells of the pig- 



