436 GASTRULATION 



thickened anterior aspect of the pit, that is, from the primitive knot 

 or primitive node (Hensen's node). 



Another peculiarity of the gastrulative procedure is found in the human 

 embryo. In the latter, precocious mesoderm is elaborated during blastulation 

 presumably from the trophoblast. Later this mesoderm becomes aggregated 

 on the inner aspect of the trophoblast layer, where it forms the internal layer 

 of the trophoblast. This precocious mesoderm gives origin to much of the 

 extra-embryonic mesoderm. However, in the majority of mammals, embryonic 

 and extra-embryonic mesoderm arise from the primitive streak as in the chick. 



6. Gastrulation in Teleost and Elasmobranch Fishes 

 a. Orientation 



Gastrulation in teleost and elasmobranch fishes shows certain similarities, 

 particularly in the fact that in both groups the migrating cells use principally 

 the dorsal-lip area of the blastopore as the gateway from the superficial layer 

 to the deeper region inside and below the superficial layer. The lateral and 

 ventral lips are used to some degree in teleosts, but the main point toward 

 which the migrating cells move is the region of the dorsal lip of the blastopore. 



As previously described (Chap. 7), the late blastular condition or blasto- 

 disc of elasmobranch and teleost fishes consists of an upper layer of formative 

 tissue, or blastodisc (embryonic disc) and a lower layer of trophoblast or 

 periblast tissue. The latter is associated closely with the yolk (figs. 179A; 

 180A; 181 A; 210A). In teleost fishes much of the presumptive entodermal, 

 organ-forming area (the so-called primary hypoblast) is represented by cells 

 which lie in the lower region of the caudal portion of the blastodisc (figs. 180A; 

 181A; 210C). The exact orientation of the hypoblast appears to vary with 

 the species. In Fundulus, a considerable amount of the presumptive entoderm 

 appears on the surface at the caudal margin of the blastodisc (fig. 180A, B). 

 (See Oppenheimer, '36.) However, in the trout, Salmo, presumptive entoderm 

 lies in the lower areas of the thickened caudal portion of the disc, and the pre- 

 chordal plate of presumptive entomesoderm alone is exposed (fig. 181 A, B). 

 (See Pasteels, '36.) The position of the presumptive entoderm in the shark, 

 Scyllium (Vandebroek, '36), resembles that of Fundulus (fig. 170A), al- 

 though some entoderm may arise by a process of delamination from the lower 

 area of the blastodisc (fig. I79A). 



b. Gastrulation in Teleost Fishes 



1) Emboly. As the time of gastrulation approaches, the entire outer edge 

 of the blastodisc begins to thicken and, thereby, forms a ring-like area around 

 the edge of the disc, known as the germ ring (figs. 2 IOC; 211B). At the 

 caudal edge of the blastoderm, the germ-ring thickening is not only more pro- 

 nounced, but it also "extends inward for some distance toward the center of 



