TELEOSTEI. 71 



marked off as a special layer (fig. 3 1 A) ; which however does 

 not constitute the whole epiblast but only a small part of it, 

 which will be spoken of as the epidermic layer. The 

 complete differentiation of the epiblast is effected by the cells of 

 the thickened edge of the blastoderm becoming divided into two 

 strata (fig. 31 B). The upper stratum constitutes the epiblast. 

 It is divided into two layers, viz., the external epidermic layer 

 already mentioned, and an internal layer known as the nervous 

 layer, formed of several rows of vertically arranged cells. 

 According to the unanimous testimony of investigators the roof 

 of the segmentation cavity is formed of epiblast cells only. The 

 lower stratum in the thickened rim of the blastoderm is several 

 rows of cells deep, and corresponds with the lower layer cells or 

 primitive hypoblast in Elasmobranchii. It is continuous at the 

 edge of the blastoderm with the nervous layer of the epiblast. 



In smaller Teleostean eggs there is formed, before the blasto- 

 derm becomes differentiated into epiblast and lower layer cells, 

 a complete stratum of cells around the nuclei in the granular 

 layer underneath the blastoderm. This layer is the hypoblast ; 

 and in these forms the lower layer cells of the blastoderm are 

 stated to become converted into mesoblast only. In the larger 

 Teleostean eggs, such as those of the Salmonidae, the hypoblast, 

 as in Elasmobranchs, appears to be only partially formed from 

 the nuclei of the granular layer. In these forms however, as in 

 the smaller Teleostean ova and in Elasmobranchii, the cells 

 derived from the granular stratum give rise to a more or less 

 complete cellular floor for the segmentation cavity. The 

 segmentation cavity thus becomes enclosed between an hypo- 

 blastic floor and an epiblastic roof several cells deep. It 

 becomes obliterated shortly after the appearance of the medul- 

 lary plate. 



At about the time when the three layers become established 

 the embryonic swelling takes a somewhat shield-like form 

 (fig- 33 A). Posteriorly it terminates in a caudal prominence 

 (ts) homologous with the pair of caudal swellings in Elasmo- 

 branchs. The homologue of the medullary groove very soon 

 appears as a shallow groove along the axial line of the shield. 

 After these changes there takes place in the embryonic layers a 

 series of differentiations leading to the establishment of the 



