368 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE 



6. Blastulae of Teleost and Elasmobranch Fishes 



In the teleost and elasmobranch fishes, the primary blastula is a flattened, 

 disc-shaped structure constructed during its earlier stages of an upper blasto- 

 derm layer of cells, the formative or strictly embryonic tissue, and a peripheral 

 and lower layer of trophoblast or periblast tissues; the latter is closely asso- 

 ciated with the yolk substance (figs. 179A; 180A; 181A). The primary blasto- 

 coelic space lies between the blastoderm and the periblast tissue. 



That margin of the formative portion of the blastoderm which lies at the 

 future caudal end of the embryo is thickened considerably, and presumptive 

 entodermal material or primary hypoblast is associated with this area. Its re- 

 lationship is variable, however. In some teleost fishes, such as the trout, the 

 entodermal cells are not exposed to the surface at the caudal portion of the 

 blastodisc (fig. 181 A; Pasteels, '36a). In other teleosts, a considerable portion 

 of the entodermal cells may lie at the surface along the caudal margin of the 

 blastoderm (fig. 180A; Oppenheimer, '36). In the elasmobranch fishes the 

 disposition of the entodermal material is not clear. A portion undoubtedly 

 lies exposed to the surface at the caudal margin of the disc (fig. 179A, B; 

 Vandebroek, '36), but some entodermal cells lie in the deeper regions of the 

 blastoderm (fig. 179A). 



Turning now to a consideration of the other presumptive organ-forming 

 areas of the fish blastoderm, we find that the presumptive pre-chordal plate 

 material lies exposed on the surface in the median plane of the future embryo 

 immediately in front of the entoderm and near the caudal edge of the blasto- 

 derm. (It is to be observed that, in comparison, the pre-chordal plate lies well 

 forward within the area pellucida of the bird blastoderm.) This condition is 

 found in the shark, Scyllium, in Fundulus, and in the trout, Sahno (figs. 

 179B; 180B). However, in the trout it lies a little more posteriorly at the 

 caudal margin of the disc (fig. 18 IB). Anterior to the pre-chordal plate is 

 the presumptive notochordal material, and anterior to the latter is a rather 

 expansive region of presumptive neural cells. These three areas thus lie along 

 the future median plane of the embryo, but they exhibit a considerable vari- 

 ation in size and in the extent of area covered in Scyllium, Fundulus, and 

 Salmo (figs. 179, 180, 181). 



Extending on either side of these presumptive organ-forming areas, is an 

 indefinite region of potential mesoderm. In Salmo, presumptive mesodermal 

 cells lie along the lateral and anterior portions of the blastoderm edge (fig. 

 18 IB). However, in Scyllium and in Fundulus, it is not as extensive (figs. 

 179B; 180B). In front of the presumptive neural organ-forming area is a 

 circular region, the presumptive epidermal area. 



In their development thus far the three blastulae described above represent 

 a primary blastular condition, and the cavity between the blastodisc and the 

 underlying trophoblast or periblast tissue forms a primary blastocoel. This 

 condition presents certain resemblances to the early blastocyst in the higher 



