170 



ZOOLOGY 



SECT. 



FIG. 7'.'4. Embryo of Scyllium canicula with the tail- 

 swellings well marked and the medullary groove just 

 beginning. 1>I. <. edge of blastoderm ; bl. p. blastopore ; 

 ><>. caudal swellings ; //. 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-vertebrce, the 

 outer part forming a 

 broad plate, the lateral 

 -plate. The splanchnic 

 layer of the proto- 

 vertebrse 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 protovertebrse, 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 



e/ 



dermis, the inter-mus- 

 cular connective tissue, 

 the endoskeleton, the 



mUSCUlar and Connective- Fic , 795._Enibryo of a Ray with the medullary groove 



tissue layers Of the all- closed except at the hind end. The notched em- 



, bryonic part of the blastoderm has grown faster than 



mentaiy Canal, tile VaS- the rest and come to project over the surface of the 



i , A i j j_v yolk. bl. e. edge of blastoderm ; hd. head ; nevr. un- 



S}'S enclosed pai't of the ncnroccele. (After Sedgwick.) 



generative organs. The 



segmentation of the mesoderm does not at first extend into the 



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 

 vulgar is, 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. 



hd. 



