TELEOSTEI. 



57 



extension of the blastoderm round the yolk, accompanied by a 

 diminution in its thickness. 





FIG. 31. LONGITUDINAL SECTIONS THROUGH THE BLASTODERM OF THE 

 TROUT AT AN EARLY STAGE OF DEVELOPMENT. 



A. at the close of the segmentation ; B. after the differentiation of the germinal layers. 

 ep'. epidermic layer of the epiblast ; sc. segmentation cavity. 



The first differentiation of the layers consists in a single row 

 of cells on the surface of the blastoderm becoming distinctly marked 

 off as a special layer (fig. 31 A) ; which however does not constitute 

 the whole epiblast but only a small part of it which Avill be spoken 

 of as the epidermic layer. The complete differentiation of the 

 epiblast is effected by the cells of the thickened edge of the blasto- 

 derm 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 investi- 

 gators 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 blastoderm 

 becomes differentiated into epiblast and lower layer cells, a com- 

 plete stratum of cells around the nuclei in the granular layer under- 

 neath 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 Salmomdse 3 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 medullary plate. 



