72 FORMATION OF THE LAYERS. 



definite organs. These changes are much more difficult to 

 follow in the Teleostei than in the Elasmobranchii, owing partly 

 to the similarity of the cells of the various layers, and partly to 

 the primitive solidity of all the organs. 



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 (fig. 32 A and B). 

 Along the dorsal line of the epiblast keel is placed the shallow 

 medullary groove ; and according to Calberla (No. 61) the keel 

 is formed by the folding together of the two sides of the 

 primitively uniform epiblastic layer. The keel becomes gradu- 

 ally 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 cerebrospinal cord- The peculiarity in the formation of 

 the central nervous system of Teleostei consists in the fact that 

 it is not formed by the folding over of the sides of the medullary 

 groove into a canal, but by the separation, below the medullary 

 groove, of a solid cord of epiblast in which the central canal is 

 subsequently formed. Various views have been put forward to 

 explain the apparently startling difference between Teleostei, 

 with which Lepidosteus and Petromyzon agree, and other verte- 

 brate forms. The explanations of Gotte and Calberla appear to 

 me to contain between them the truth in this matter. The 

 groove above in part represents the medullary groove ; but the 

 closure of the groove is represented by the folding together 

 of the lateral parts of the epiblast plate to form the medullary 

 keel. 



According to Gotte this is the whole explanation, but Calberla states for 

 Syngnathus and Salmo that the epidermic layer of the epiblast is carried 

 down into the keel as a double layer just as if it had been really folded in. 

 This ingrowth of the epidermic layer is shewn in fig. 32 A where it is just 

 commencing to pass into the keel ; and at a later stage in fig. 32 B where 

 the keel has reached its greatest depth. 



