676 THE HEAD-CAVITIES. 



Whether the muscle of the diaphragm is to be placed in the same 

 category as the hyposkeletal muscles has not been made out. 



It is probable that the cutaneous muscles of the trunk are derived 

 from the cells given off from the muscle-plates. Kolliker however believes 

 that they have an independent origin. 



The limb-muscles, both extrinsic and intrinsic, as may be concluded 

 from their development in Elasmobranchii, are derived from the muscle- 

 plates. Kleinenberg found in Lacertilia a growth of the muscle-plates 

 into the limbs, and in Amphibia Gotte finds that the outer layer of the 

 muscle-plates gives rise to the muscles of the limbs. 



In the higher Vertebrata on the other hand the entrance of the muscle- 

 plates into the limbs has not been made out (Kolliker). It seems therefore 

 probable that by an embryological modification, of which instances are so 

 frequent, the cells which give rise to the muscles of the limbs in the higher 

 Vertebrata can no longer be traced into a direct connection with the muscle- 

 plates. 



TJte Somites and muscular system of the head. 



The extension of the somites to the anterior end of the body 

 in Amphioxus clearly proves that somites, similar to those of 

 the trunk, were originally present in a region, which in the 

 higher Vertebrata has become differentiated into the head. In 

 the adult condition no true Vertebrate exhibits indications of 

 such somites, but in the embryos of several of the lower Verte- 

 brata structures have been found, which are probably equivalent 

 to the somites of the trunk : they have been frequently alluded 

 to in the previous chapters of this volume. These structures 

 have been most fully worked out in Elasmobranchii. 



The mesoblast in Elasmobranch embryos becomes first split 

 into somatic and splanchnic layers in the region of the head ; 

 and between these layers there are formed two cavities, one on 

 each side, which end in front opposite the blind anterior ex- 

 tremity of the alimentary canal ; and are continuous behind 

 with the general body-cavity (fig. 20 A, vp}. I propose calling 

 them the head-cavities. The cavities of the two sides have 

 no communication with each other. 



Coincidently with the formation of an outgrowth from the 

 throat to form the first visceral cleft, the head-cavity on each 

 side becomes divided into a section in front of the cleft and a 

 section behind the cleft ; and at a later period it becomes, owing 

 to the formation of a second cleft, divided into three sections : 



