LATE CLEAVAGE THROUGH GASTRULATION 



119 



Fig. 61. Salamander gastrulae corresponding to stages 11 and 12, Figure 58. 

 Left: An almost perfect semicircular lip of the blastopore, stage 11. A diffuse 

 dark line completes a circle. The upper half of the circle is a rather sharp line. 

 Cells above and to the sides of this line are moving toward the line and disap- 

 pearing into the interior to form the roof and sides of the archenteron. Although 

 the blastopore is continuous, we nevertheless distinguish a dorsal lip from two 

 lateral lips. Obviously there is no sharp demarcation between dorsal and lateral 

 lips. The lower half of the circle is not yet a part of the blastopore, but the rela- 

 tively dark region foreshadows the ventral lip of the blastopore. The mass of 

 cells inside the circle is presumptive endoderm. Right: A circular blastopore with 

 its lips engulfing practically all the presumptive endoderm. The white area inside 

 the circle is formed of yolk cells and is therefore termed a yolk plug. This plug 

 fits tightly into the blastopore. (Courtesy Professor John A. Moore, Columbia 

 University.) 



in size at 24 and 28 hours. On each side of the dorsal lip more and more cells 

 become involved in the process of gastrulation, and the blastoporal lip 

 extends laterally, forming a semicircle (Fig. 58, stage 11; see also Fig. 61). 

 The lateral lips extend farther ventrally and meet to form the ventral lip of 

 the blastopore (stage 12, Fig. 58 and 61). This blastopore communicates with 

 the archenteron but the lips fit tightly over the yolk, and thus an actual open- 

 ing of the archenteron to the exterior is not present at this time. The yolk 

 forms a plug which fits tightly in the blastopore. As the archenteron becomes 

 larger, the blastocoel becomes smaller and finally is completely obliterated. 

 During development from 6 to 28 hours there has been little change in 

 shape, and the gastrula is almost spherical. There has been some increase in 



