GASTRULATION PROPER 119 



tion of the adjacent portions of the marginal zone. This insures that the 

 presumptive chorda is in a position to exert a double effect by means of its 

 extension, for it will not only pull the lateral marginal zones dorsaiward, 

 but will also carry them forward in a dorsal position. Meanwhile the blasto- 

 poral lips are constricted, since there is progressive withdrawal of marginal 

 zone material by the process of dorsaiward convergence somewhat as the 

 mouth of a purse is constricted when the purse string is pulled. The tend- 

 ency of constriction is augmented further by the forward migration of the 

 internal portions of the marginal zone, for this movement also tends to 

 withdraw material from the region of the blastoporal lips. 



In summary, Schechtman believes: 



1. The presumptive chorda is invaginated by the inwardly di- 

 rected tension or pull exerted by the invagination and involu- 

 tion of the lateral marginal zone, with which it is continuous. 

 • 2. The lateral marginal zones are then pulled dorsaiward and 

 inward in the dorsal position by the autonomous stretching 

 and simultaneous narrowing of the presumptive chorda. 

 3. The constriction of the blastoporal lips over the yolk mass is 

 affected by the progressive withdrawal of marginal zone ma- 

 terial by dorsaiward convergence. 



Internally the involuted cells extend anteriorly, away from the 

 point of involution or the dorsal lip of the blastopore. The inturned 

 endoderm surrounds the new cavity, which is at first no more than a 

 slit. The cavity (archenteron) rapidly expands in all directions so 

 that the blastocoel, originally in a dorsal position, becomes progres- 

 sively displaced anteriorly or away from the side of the blastopore 

 formation. The blastocoel is later displaced antero-ventrally by this 

 expanding archenteron. It also becomes reduced in size until finally 

 it is found only as the slit between the endoderm and the yolk, referred 

 to as the gastrular slit. Sometimes there is a remnant of the blastocoel, 

 separated from the archenteron by a single layer of cells called the 

 completion bridge. This bridge is of no consequence, and frequently 

 ruptures to merge the contents of the blastocoel and the gastrocoel. 



These movements of involuting cells and expanding endoderm 

 result in an enlarged archenteron (gastrocoel), entirely lined with 

 endoderm. As stated, the roof and lateral wall cells are more pig- 

 mented than the yolk endoderm floor cells of the archenteron. The 

 opening beneath the dorsal lip of the blastopore, and into the archen- 

 teron, is called the blastopore. This is an incorrect term, however, for 



