33O DEVELOPMENT OF ELASMOBRANCH FISHES. 



a result of this the primitive continuity of the body-cavity and 

 cavity of the muscle-plates becomes to a certain extent masked, 

 though its presence may easily be detected by the obvious 

 continuity which at first exists between the somatic and splanch- 

 nic layers of mesoblast and the two layers of the muscle-plate. 

 In the muscle-plate itself the chief point to be noticed is the fact 

 that the earlier formed bands of muscles (mp 1 ) arise very much 

 later, and are less conspicuous, in Torpedo than in the genera 

 first described. They are however present and functional. 



The anatomical relations of the body-cavity itsejf are pre- 

 cisely the same in Torpedo as in Pristiurus and Scyllium, and 

 the pericardial cavity becomes separated from the peritoneal in 

 the same way in all the genera ; the two lateral canals connect- 

 ing the two cavities being also present in all the three genera. 

 The two independent parietal plates of mesoblast of the posterior 

 parts of the body have ventrally a swollen edge, as in Pristiurus, 

 and in this a cavity appears which forms a posterior continuation 

 of the true body-cavity. 



Resume. The primitive independent mesoblast plates of the 

 two sides of the body become divided into two layers, a somatic 

 and a splanchnic (Hautfaserblatt and Darmfaserblatt). At the 

 same time in the dorsal part of the mesoblast plate a series of 

 transverse splits appear which mark out the limits of the proto- 

 vertebrae and serve to distinguish a dorsal or vertebral part of the 

 plate from a ventral or parietal part. 



Between the somatic and splanchnic layers of the mesoblast 

 plate a cavity arises which is continued quite to the summit 

 of the vertebral part of the plate. This is the primitive body- 

 cavity ; and at first the cavity is divided into two lateral and 

 independent halves. 



The next change which takes place is the complete separa- 

 tion of the vertebral portion of the plate from the parietal ; 

 thereby the upper segmented part of the body-cavity becomes 

 isolated and separated from the lower and unsegmented part. 

 In connection with this change in the constitution of the body- 

 cavity there are formed a series of rectangular plates, each com- 

 posed of two layers, a somatic and a splanchnic, between which 

 is the cavity originally continuous with the body-cavity. The 

 splanchnic layer of the plates buds off cells to form the rudi- 



