CO FORMATION OF THE LAYERS. 



to the former, while the auditory and olfactory vesicles are continuoits with 

 the latter. 



After its separation from the central nervous system the remainder 

 of the epiblast gives rise to the skin, etc., and most probably the 

 epidermic stratum develops into the outer layer of the epidermis and 

 the nervous stratum into the mucous layer. The parts of the organs 

 of special sense, which arise from the epiblast, are developed from 

 the nervous layer. In the Trout (Oellacher, No. 72) both layers are 

 continued over the yolk-sack ; but in Clupeus and Gasterosteus only 

 the epidermic has this extension. According to Gotte the distinc- 

 tion between the two layers becomes lost in the later embryonic 



stages. 



Although it is thoroughly established that the mesoblast ori- 

 ginates from the lower of the two layers of the thickened embryonic 

 rim, it is nevertheless not quite certain whether it is a continuous 

 layer between the epiblast and hypoblast, or whether it forms two 

 lateral masses as in Elasmobranchs. The majority of observers take 

 the former view, while Calberla is inclined to adopt the latter. 

 In the median line of the embryo underneath the medullary groove 

 there are undoubtedly from the first certain cells which eventually 

 give rise to the notochord ; and it is these cells the nature of which 

 is in doubt. They are certainly at first very indistinctly separated 

 from the mesoblast on the two sides, and Calberla also finds that 

 there is no sharp line separating them from the secondary hypoblast 

 (fig. 32 A). Whatever may be. the origin of the notochord the 

 mesoblast very soon forms two lateral plates, one on each side of 

 the body, and between them is placed the notochord (fig. 32 B). 

 The general fate of the two mesoblast plates is the same as in Elas- 

 mobranchs. They are at first quite solid and exhibit relatively late 

 a division into splanchnic and somatic layers, between which is 

 placed the primitive body cavity. The dorsal part of the plates 

 becomes transversely segmented in the region of the trunk ; and thus 

 gives rise to the mesoblastic somites, from which the muscle plates 

 and the perichordal parts of the vertebral column are developed. The 

 ventral or outer part remains unsegmented. The cavity of the ventral 

 section becomes the permanent body cavity. It is continued forward 

 into the head (Oellacher), and part of it becomes separated off 

 from the remainder as the pericardial cavity. 



The hypoblast forms a continuous layer below the mesoblast, and, 

 in harmony with the generally confined character of the development 

 of the organs in Teleostei, there is no space left between it and the 

 yolk to represent the primitive alimentary cavity. The details of the 

 formation of the true alimentary tube have not been made out ; it is 

 not however formed by a folding in of the lateral parts of the hypo- 

 blast, but arises as a solid or nearly solid cord in the axial line, 

 between the notochord and the yolk, in which a lumen is gradually 

 established. 



