302 DEVELOPMENT OF ELASMOBRANCH FISHES. 



Coincidentally with the appearance of a differentiation into 

 a somatic and splanchnic layer the mesoblast plates become 

 partially split by a series of transverse lines of division into pro- 

 tovertebrae. Only the proximal regions of the plates become 

 split in this way, while their peripheral parts remain quite intact. 

 As a result of this each plate becomes divided into a proximal 

 portion adjoining the medullary canal, which is divided into 

 protovertebra, and may be called the vertebral plate, and a 

 peripheral portion not so divided, which may be called the 

 lateral plate. These two parts are at this stage quite continuous 

 with each other ; and, as will be seen in the sequel, the body- 

 cavity originally extends uninterruptedly to the summit of the 

 vertebral plates. 



By stage D at the least ten protovertebrae have appeared. 

 In Torpedo the mesoblast commences to be divided into two 

 layers much earlier than in Pristiurus ; and even before stage C 

 this division is more or less clearly marked. 



In the head and tail the condition of the mesoblast is by no 

 means the same as in the body. 



In the tail the plates of mesoblast become considerably 

 thickened and give rise to two projections, one on each side, 

 which have already been alluded to as caudal or tail-swellings ; 

 vide PI. 8, figs. D, F, and PI. 10, fig. 3/and fig. 4 ts. 



These masses of mesoblast are neither divided into proto- 

 vertebrae, nor do they exhibit any trace of a commencing dif- 

 ferentiation into somatopleure and splanchnopleure. 



In the head, so far as I have yet been able to observe, the 

 mesoblastic plates do not at this stage become divided into 

 protovertebrse. The other changes exhibited in the cephalic 

 region are of interest, mainly from the fact that here appears a 

 cavity in the mesoblast directly continuous with the body-cavity 

 (when that cavity becomes formed), but which appears at a 

 very much earlier date than the body-cavity. This cavity can 

 only be looked on in the light of a direct continuation of the 

 body or peritoneal cavity into the head. Theoretical considera- 

 tions with reference to it I propose reserving till I have described 

 the changes which it undergoes in the subsequent periods. 



PI. 10, figs. 3 a, 3 b and 3 c exhibit very well the condition 

 of the mesoblast in the head at this period. In fig. 3 c, a section 



