52 WORK IN THE HATCHING-HOUSE. 



stratum in the thickened rim of the blastoderm is several rows of 

 cells deep, and corresponds with the lower-layer cells, or the primi- 

 tive hypoblast in Elasmobranchii. It is continuous at the edge of 

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

 In the lai-ger Teleostean eggs, such as those of the Salmonidce, 

 the hypoblast, as in Elasmobranchs, appears to be only partially 

 formed from the nuclei of the granular layer. . . . The cells 

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

 plete cellular floor for the segmentation cavity. The segmenta- 

 tion cavity thus becomes enclosed between an hypoblastic floor 

 and an epiblastic roof several cells deep ; it becomes obliterated 

 shortly after the appearance of the medullary plate. 



" The first changes in the epiblast give rise to the central nervous 

 system. The epiblast, consisting of the nervous and epidermic 

 strata already indicated, becomes thickened along the axis of the 

 embryo, and forms a keel projecting towards the yolk below. So 

 great is the size of this keel in the front part of the embryo that 

 it influences the form of the whole body, and causes the outline 

 of the surface adjoining the yolk to form a strong ridge moulded 

 on the keel of the epiblast. Along the dorsal line of the epiblast 

 keel is placed the shallow medullary groove. The keel becomes 

 gradually constricted off from the external epiblast, and then forms 

 a solid cord below it. Subsequently there appears in this cord 

 a median slit-like canal, which forms the permanent central canal 

 of the cerebro-spinal cord. The separation of the solid nervous 

 system from the epiblast takes place relatively very late, and 

 before it has been completed the first traces of the auditory pits, 

 of the optic vesicles, and of the olfactory pits are visible. The 

 auditory pit arises as a solid thickening of the nervous layer of the 

 epiblast at its point of junction with the medullary keel, and the 

 optic vesicles spring as solid outgrowths from part of the keel 

 itself ; the olfactory pits are barely indicated as thickenings of the 

 nervous layer of the epiblast. 



" 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 



