PHYLUM CHORDATA 



185 



Fro. S42. Embryo of Scyllium canicula with the tail- 

 swellings well marked and the medullary groove just 

 beginning. U. e. edge of blastoderm ; M. p. blastopore ; 

 r</. caudal swellings ; hd. head. (After Sedgwick.) 



lateral plates eventually unite ventrally, and their cavities coalesce 

 to form the body-cavity. The parts derived from the mesoderm 

 are the system of voluntary muscles, the dermis, the inter-muscular 

 connective-tissue, the 

 endoskeleton, the mus- 

 cular and connective- 

 tissue layers of the 

 alimentary canal, the 

 vascular system, and 

 the generative organs. 

 The segmentation of 

 the mesoderm does 

 not at first extend 

 into the head, but, 

 on the formation of 



the gill-clefts, a series "*> -^T^ **~ bLe, 



of mesodermal seg- 

 ments appear, the cells 

 of which give rise to 

 the muscles of the 

 branchial, hyoid, and 



mandibular arches, and probably also of the palatoquadrate and 

 the eye. 



By degrees the body of the young Fish becomes moulded on the 

 blastoderm. This is effected by the formation of a system of folds, 



anterior, posterior, and 

 lateral, which grow in- 

 wards in such a way as 

 to separate off the body 

 of the embryo from the 

 rest of the blastoderm 

 enclosing the yolk. As 

 the folds approach one 

 another in the middle, 

 underneath the embryo, 

 they come to form a con- 

 striction connecting the 

 body of the embryo with 

 the yolk enclosed in the 

 extra-embryonic part of 

 the blastoderm. The pro- 

 cess may be imitated if 



FIG. 843. Embryo of a Ray with the medullary groove 

 closed except at the hind end. The notched em- 

 bryonic part of the blastoderm has grown faster than 

 the rest and come to project over the surface of the 



, yolk. bl. e. edge of blastoderm ; hd. head ; neur. un- 

 enclosed part of the neurocoele. (After Sedgwick.) 



necting the pinched-off portion with 



we pinch off a portion 

 of a ball of clay, leaving 

 only a narrow neck con- 

 the rest. The body of 



the embryo thus becomes folded off from the yolk-sac and comes to 



