170 



ZOOLOGY 



SECT. 



FIG. 794. Embryo of Scyllium canicula with the tail- 

 swellings well marked and the medullary groove just 

 beginning, bl. c. edge of blastoderm ; bl. p. blastopore ; 

 cd. caudal swellings ; hd, head. (After Sedgwick.) 



The notochord (Fig. 793, ch.) is developed as a cord of cells 

 derived from the lower layer. 



Each of the two plates of mesoderm soon divides into two 



layers, somatic and 

 splanchnic, with a 

 cavity between them. 

 The inner part of each 

 separates off from the 

 rest and becomes 

 divided by transverse 

 fissures into a series 

 of squarish masses, 

 the proto-vertcbrce, the 

 outer part forming a 

 broad plate, the lateral 

 plate. The splanchnic 

 layer of the proto- 

 vertebras sends off cells 

 round the notochord 

 to form the bodies of 

 the vertebrae, the re- 

 mainder giving rise to the muscles of the voluntary system. The 

 isthmus between the lateral plate and the protovertebra?, con- 

 taining a prolongation of 

 the cavity, gives rise to 

 the pronephric duct and 

 tubules. The lateral 

 plates eventually unite 

 ventrally, and their cavi- 

 ties coalesce to form the 

 body cavity. The parts 

 derived from the meso- 

 derm are the system of 

 voluntary muscles, the 

 dermis, the inter-mus- 

 cular connective tissue, 

 the endoskeleton, the 

 muscular and connective- 

 tissue layers of the ali- 

 mentary canal, the vas- 

 cular system, 1 and the 

 generative organs. The 

 segmentation of the mesoderm does not at first extend into the 



FIG. 795. 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 neuroccele. (After Sedgwick.) 



1 There is some uncertainty as to the germinal layer from which the internal 

 lining membrane of the heart and blood-vessels is derived. In Acanthias 

 vidgaris, if not in others, it seems to be derived from the endoderm, and the entire 

 vascular system is to be looked upon as a separated-off part of the archenteron. 



