58 FORMATION OF THE LAYERS. 



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 Elasmobranchs. 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 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 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 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 vertebrate 

 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. 



Gotte maintains that Calberla's statements are not to be trusted, and 

 I have myself been unable to confirm them for Teleostei or Lepidosteus ; 

 but if they could be accepted the difference in the formation of the 

 medullary canal in Teleostei and in other Vertebrata would become 



