412 



GASTRULATION 



ANTERIOR 



SMALL 

 INTESTINE 



NEURAL ECTODERM 

 BRAIN AREA 

 NOTOCHORD 



STOMACH 



PdTERIOR 



l\ FOREGUT 



BLASTOPORAL 



LIP 



ESOPHAGUS 



YOLK 



^ EPIDERMAL 



ECTODERM 



V E N T R A L 



MESODERM 



Fig. 194. Developmental tendencies of entodermal area and their reorientation during 

 gastrulation. (A) Developmental tendencies of entodermal area of young anuran gas- 

 trula. (B) Counterclockwise rotation of approximately 90° of the entodermal area 

 during gastrulation. 



together with the entoderm and yolk pass to the inside, the forces involved 

 in epiboly effect the expansion of the purely ectodermal portion of the epiblast 

 which gradually comes to cover the entire external surface of the gastrula with 

 the exception of the immediate blastoporal area (study black and white areas 

 in fig. 191A-E). It may be observed further that the neural crescent now is 

 elongated along the antero-posterior, embryonic axis where it forms a shield- 

 shaped region with the broad end of the shield located anteriorly (fig. I92A). 



A study of figure 19 IE and F shows that a rotation of the entire gastrula 

 occurs in the interim between E and F. This rotation is induced by the inward 

 movement of the entoderm and yolk, depicted in figure 191C-E, with a 

 subsequent shift in position of the heavy mass of yolk from the posterior pole 

 of the embryo to the embryo's ventral or belly region. Most of the blastocoel 

 and its contained fluid is "engulfed" by the inward moving entoderm, as indi- 

 cated in figure 191C-E, some of the blastocoelic fluid and blastocoelic space 

 passes over into the gastrocoel. The region of the entodermal yolk mass shown 

 to the left in figure 19 IE, therefore, is more dense and heavier than the area 

 shown to the right. The heavier region of the gastrula seeks the lower level; 

 hence the rotation of the entire gastrula, and the new position assumed in 

 figure 19 IF. 



As the blastopore progressively grows smaller, it eventually assumes a small, 

 rounded appearance (fig. 197A-E), and the remnants of the presumptive 

 mesoderm pass over the lips of the blastopore before it closes. In doing so, the 

 presumptive tail mesoderm converges dorsally and becomes located inside 

 the dorso-lateral portion of the closed blastopore near the lateral aspects of the 

 posterior end of the folding neural plate. 



A short while previous to blastoporal closure, the midregion of the neural 

 plate area begins to fold ventrad toward the notochord, while its margins are 



