GASTRULATION IN VARIOUS CHORDATA 415 



organizer (fig. 191E-G). Eventually pre-chordal plate cells contribute to the 

 pharyngeal area of the foregut and give origin to a portion of the head meso- 

 derm, at least in many vertebrate species (Chap. 11, p. 523). 



c. Closure of the Blastopore and Formation of the Neurenteric Canal 



The closure of the blastopore and formation of the neurenteric canal is 

 described in Chapter 10, p. 471. 



d. Summary of Morphogenetic Movements of Cells During Gastrulation 



in the Frog and Other Amphibia 

 1) Emboly: 



(a) Invagination. Invagination in the Amphibia appears to consist of two 

 phases: ( 1 ) an active infolding or forward migration of the future 

 foregut, stomach, etc., areas, and (2) an insinking and inward rotation 

 of future intestinal and heavily laden, yolk cells. 



(b) Convergence. This activity is found in the presumptive, notochordal 

 and mesodermal cells as they move toward the blastoporal lips. A 

 dorsal convergence toward the dorsal, blastoporal-lip area is particu- 

 larly true of the more laterally placed parts of the notochordal crescent 

 and to some extent also of the somitic and lateral plate mesoderm. 

 The tail mesoderm tends to converge toward the dorsal blastoporal 

 area when the blastopore nears closure. 



(c) Involution. An inward rolling or rotation of cells over the blastoporal 

 lips to the inside is a conspicuous part of notochordal and mesodermal 

 cell migration. 



(d) Divergence. After the mesodermal cells have migrated to the inside, 

 there is a particular tendency to diverge on the part of the lateral plate 

 and ventral mesoderm. The lateral plate mesoderm diverges laterally 

 and ventrally, while the ventral mesoderm diverges laterally in the 

 ventral or belly area of the gastrula. 



(e) Extension. The phenomenon of extension or elongation is a charac- 

 teristic feature of all gastrulative processes in the chordate group. Be- 

 fore arriving at the blastoporal lips, the converging notochordal and 

 mesodermal cells may undergo a stretching or extending movement. 

 That is, convergence and stretching are two prominent movements in- 

 volved in the migration of the marginal zone or chordamesodermal 

 cells as they move toward the blastoporal lip. After these materials 

 have involuted to the inside, the chordal cells stretch antero-posteriorly 

 and become narrowed to a cuboidal band in the midline, and the 

 lateral plate mesoderm stretches anteriorly as it diverges laterally. 

 Antero-posterior extension of the somitic mesoderm also occurs. 



(f ) Contractile tension or constriction. A considerable constriction or con- 

 traction around the edges of the blastopore occurs as gastrulation pro- 



