GASTRULATION IN VARIOUS CHORDATA 439 



germ-ring tissue, together with the presumptive epidermal area, proceeds to 

 expand rapidly. This growth and expansion soon bring about an engulfment 

 of the yolk mass (figs. 210B; 211C-F). The blastoporal-hp area (i.e., edge of 

 germ ring) ultimately fuses at the caudal trunk region (figs. 210G; 21 IF). 

 As the blastoporal region becomes narrower, a small vesicular outpocketing, 

 known as Kupflfer's vesicle, makes its appearance at the ventro-caudal end of 

 the forming embryo at the terminal end of the solid, post-anal gut (fig. 210G). 

 This vesicle possibly represents a vestige of the enteric portion of the neuren- 

 teric canal found in Amphioxus, frog, etc. A certain amount of mesodermal 

 involution occurs around the edges of the germ ring, in some species more 

 than in others (fig. 210A, B, peripheral mesodermal involution). 



As the cellular dispositions involved in extra-embryonic expansion of the 

 epidermal and germ-ring areas are established, the presumptive, neural plate 

 material (figs. 179, 180, 181) becomes greatly extended antero-posteriorly 

 in the dorsal midline (figs. 210A, H; 211E), where it forms into a thickened, 

 elongated ridge or keel. The latter gradually sinks downward toward the 

 underlying notochordal tissue (fig. 210E, F). Also, by the time that the yolk 

 mass is entirely enveloped, the somites appear within the mesoderm near the 

 notochordal axis, and the developing body as a whole may be considerably 

 dehmited from the surrounding blastodermic tissue (fig. 21 IG). Therefore, 

 if the envelopment of the yolk mass is taken as the end point of gastrulation 

 in teleosts, the stage at which gastrulation is completed does not correspond 

 to the developmental condition found at the termination of gastrulation in the 

 chick, frog, and other forms. That is, the embryo of the teleost fish at the 

 time of blastoporal closure is in an advanced stage of body formation and 

 corresponds more truly with a chick embryo of about 35 to 40 hours of incu- 

 bation, whereas the gastrulative processes are relatively complete in the chick 

 at about 20 to 22 hours of incubation. 



3) Summary of the Gastrulative Processes in Teleost Fishes: 



a) Emboly: 



( 1 ) Formation of the secondary hypoblast. The secondary hypoblast forms 

 as a result of the forward migration, expansion, and proliferation of 

 the entodermal cells lying at the caudal margin of the embryonic shield. 

 This forward migration of the entoderm (primary hypoblast) occurs 

 below the upper layer or epiblast and thus produces an underlying 

 entodermal layer or secondary hypoblast. 



(2) Pre-chordal plate and notochordal involution. As the formation of 

 the secondary hypoblast is initiated, the presumptive pre-chordal plate 

 and notochordal cells move posteriad and converge toward the dorsal 

 lip of the blastopore, where they involute and pass anteriad in the 

 median line between the hypoblast and epiblast. The hypoblast or 

 entodermal layer may be separated into two flattened layers or lamellae, 

 one on either side of the notochord in some species. However, there 



