PISCES 163 



the embryo, the ectoderm being thickened and the somatic 

 mesoderm multiplied. These are accompanied by the forma- 

 tion of buds of the myotomes, which yield an abundant 

 mesoderm and ultimately the muscles of the limb in each 

 case (fig. 93). 



The pectoral fins are connected by a girdle of cartilage. 

 The pectoral girdle consists of a dorsal scapula and supra- 

 scapula on each side, the suprascapulae almost meeting to be 

 attached to the expanded crest of the fused region of the 

 vertebral column. The scapula on each side, after giving 

 articulation to the elements of the fin, passes below into a bar 

 which extends across the ventral body wall to meet and become 

 continuous with the scapula of that side. This is the cora- 

 coid. Three facets are provided on each side for the articu- 

 lation of the pro-, meso-, and meta-pterygia, and to these the 

 long fine fin rays are articulated. 



The pelvic girdle consists of a bar of cartilage which lies 

 transversely in the ventral body wall at the hinder end of the 

 body. This is the ischial bar, and at each end it is produced 

 into an anterior pubic and an upwardly directed iliac process. 

 At each end also two facets are provided for the rays and the 

 metapterygium. In the male, as has already been observed, 

 the inner part of the fin is highly modified and enlarged to 

 form the clasper on each side. 



The median fins are supported by basal plates which 

 connect them to the caudal vertebrae. 



Alimentary Canal. The alimentary canal in all vertebrates 

 is almost entirely derived from endoderm. It is only the 

 anterior part of the mouth which has a stomodeal origin, and 

 only a part of the cloaca which has a proctodeal origin. 



In the skate the cartilaginous jaws bear numerous teeth 

 in parallel transverse rows. They are renewed on the inner 

 side, gradually rotated to the opposing surface and lost on the 

 outer surface. The teeth are developed as the placoid scales. 

 But in this case the formation is associated with the appearance 

 of a prolongation of the inner ectoderm of the mouth into 

 the dermis as a dental band. The dental band internally is 

 formed into cups, each of which encloses a cone of dermis. The 

 dermal cells next the ectodermal cup, or odontoblasts, proceed 



