GASTRULATION IN VARIOUS CHORDATA 403 



plate is initiated, notochordal cells in the middorsal region of the blastopore 

 involute, move inward along with the entoderm, and come to occupy a position 

 in the middorsal area of the forming archenteron (fig. 190C-E). Similarly, 

 mesodermal cells in the upper or dorsal ends of the mesodermal crescent 

 gradually converge dorso-mediad and pass into the roof of the forming gas- 

 trocoel (archenteron) on either side of the median area occupied by the 

 notochordal cells (fig. 190F, G). Thus the roof of the gastrocoel is composed 

 of notochordal and mesodermal cells (fig. 195A, B), 



2) Epiboly. As the above events come to pass, the potential epidermal 

 and neural cells proliferate actively, and both areas gradually become extended 

 in an antero-posterior direction. In this way the neural ectoderm becomes 

 elongated into a median band which lies in the middorsal region of the gastrula 

 (figs. 190A-H; 247B-F), while the epidermal area covers the entire gastrula 

 externally with the exception of the neural area. 



Thus, the general result of this proliferation, infolding, and involution of 

 the presumptive entodermal, notochordal, and mesodermal cells, together 

 with the extension and proliferation of the ectodermal cells is the production 

 of a rudimentary double-layered embryo or gastrula (figs. 189, 190). Ecto- 

 dermal cells (epidermal and neural) form the external layer (fig. 190G). 

 The internal layer is composed of notochordal cells in the dorso-median area 

 with two narrow bands of mesodermal cells lying along either side of the 

 median notochordal band of cells while the remainder of the internal layer 

 is composed of entodermal cells (figs. 190G; 195A, B). At the blastoporal 

 end of this primitive gastrula are to be found proliferating notochordal, meso- 

 dermal, entodermal, and ectodermal cells. 



3) Antero-posterior Extension of the Gastrula and Dorsal Convergence of 

 the Mesodermal Cells. The processes associated with epiboly bring about an 

 antero-posterior extension of the ectodermal layer of cells. Similarly, the cells 

 which are moved inward by embolismic forces are projected forward toward 

 the future cephalic end of the embryo and become extended along the median 

 embryonic axis. Epiboly and emboly, accompanied by rapid cell proliferation 

 at the blastoporal-lip area, thus effect an antero-posterior elongation of the 

 developing gastrula (figs. 189H; 190H). 



As the gastrula is extended in the antero-posterior direction, a shift occurs 

 in the position of the mesodermal cells which form the ventral or mesodermal 

 crescent. The ventral crescent becomes divided ventrally into two halves, and 

 each half gradually moves dorsalward along the inner aspect of the lateral 

 blastoporal lips as gastrulation is accomplished. Each arm of the original 

 crescent in this manner converges dorso-mediad toward the median noto- 

 chordal cells of the dorsal blastoporal lip, and a mass of mesodermal cells 

 comes to lie along either side of the notochordal cells. As a result of this 

 converging movement, entodermal cells of the blastoporal area converge dorso- 



