GASTRULATION PROPER 



Animal pole 



111 



Sperm entrance 

 point 



Ventral 



side \ io»| 



30° \ 



Limit of involution 



L Dorsal 



r side 



Ventral blastopore lip 



Dorsal blastopore lip 

 Gray crescent 

 Vegetal pole 

 Surface changes during gastrulation. (Modified from Pasteels.) 



of the blastula away from the original site of the gray crescent. 

 This is the region where involution will first take place, the 

 region which will come to be known as the dorsal lip of the 

 blastopore. The blastula wall which is equatorially opposite to 

 the gray crescent side may, in consequence, become several 

 cells thicker. 

 2. Continued epiboly or extension of the marginal cell zone toward 

 the vegetal hemisphere , so that the diameter of the marginal 

 zone becomes progressively smaller as it passes below the equa- 

 tor. The marginal zone attains increasing rigidity so that it 

 exerts an inward pressure on the yolk cells which are being en- 

 circled, causing them to be arched upward toward the blastocoel. 

 One might draw the simile of an overlapping rubber membrane 

 being drawn down over a mass of slightly less resistant mate- 

 rial, toward the center of which there is a cavity. The yolk 



{Continued from opposite page.) 

 (C) Blastoporal view of successive phases of gastrulation; (solid line) lip of 

 blastopore, {dotted line) germ ring, to be subsequently incorporated into the 

 blastoporal lips. (D) Lateral view of sagittal section during late gastrulation 

 showing the origin of the mesial notochord, and the lateral mesoderm from the 

 proliferated chorda-mesoderm cells at the dorsal lip. (E) Composite drawing to 

 illustrate the germ layer relations in the later gastrula of the frog. The medullary 

 plate (ectoderm) is not indicated; {alternate dots and dashes) notochord, {heavy 

 stippling) notochord, {sparse stippling) mesoderm, (cellular markings) ecto- 

 derm. 



