112 EESUME. 



and Scy Ilium. In some cases even the muscle-plates become 

 definitely separated and independent before the true body- 

 cavity has appeared. As a result of this the primitive con- 

 tinuity 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 splanchnic 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) 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 itself are pre- 

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

 the pericardial cavity becomes separated from the peritoneal 

 in 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 continua- 

 tion 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- 

 vertebra3 and serve to distinguish a dorsal or vertebral part of 

 the plate from a ventral or parietal jDart. 



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- 



