GASTRULATION PROPER 121 



the pore opens into the gastrocoel and not into the blastocoel. The 

 blastopore is occluded by the yolk endoderm cells but later opens 

 into the perivitelline space by the withdrawal of the yolk plug. It will 

 close ultimately with the development of tail mesoderm. 



The reduction of the blastocoel and the enlargement of the archen- 

 teron together alter the gravitational axis of the entire embryonic 

 mass, shifting it about 90" away from the position of the blastopore. 

 The yolk plug, instead of remaining in a ventral position, comes to 

 lie about the level of the original equator. This all occurs while 

 the embryo lies free within its fertilization membrane. Further, the 

 presence of the archenteric roof and the chorda-mesoderm cells 

 above it cause the overlying ectoderm to thicken and the whole embryo 

 becomes elongated in an antero-posterior direction, more or less 

 horizontal to the revised center of gravity. The yolk plug now marks 

 the posterior region of the future embryo. 



It must be emphasized that, during this shift in the gravitational 

 axis, there has not been a comparable shifting of the internal relation- 

 ships of the embryo. The blastopore and all related parts of the em- 

 bryo have been rotated dorsally, within the enveloping fertilization 

 membrane. The axis of the gastrula is simply altered by the develop- 

 ment of this new cavity, the archenteron, with its consequent oblitera- 

 tion of the blastocoel. 



