xm 



PHYLUM CHORDATA 



179 



bl.e, 



FIG. 853. Embryo of Scyllium canicula with the tail- 

 swellings well marked and the medullary groove just 

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

 cil. caudal swellings ; M. head. (After Sedgwick.) 



The notochord (Fig. 852, cli.) is developed as a cord of cells 



derived from the lower layer. 



Each lateral sheet of mesoderm, soon after its formation by the 



coalescence of the peripheral and central rudiments, becomes divided 



by the development of 



a horizontally directed 



cleft-like space in its 



interior. The inner part 



of each sheet then sepa- 

 rates from the outer by 



the formation of a longi- 

 tudinal fissure. The 



former, which is known 



as the vertebral plate, 



becomes divided by 



transverse fissures into 



a number of squarish 



masses, the protovertebrcB 



or mesodermal somites. 



The outer part forms a 



broad plate, the lateral 



plate. The lateral plate consists of two layers, a dorsal or somatic, 



and a ventral or splanchnic, and the cavity between them is the 



beginning of the ccelome. The protovertebrae send off cells round 



the notochord to form the bodies of the vertebrae, the remainder 



giving rise to the muscles 

 of the voluntary system. 

 An isthmus of mesoderm 

 cells (neplirotome), which 

 still connects each proto- 

 vertebra with the lateral 

 plate and contains a pro- 

 longation of the cavity, 

 gives rise to the pronephric 

 duct and tubules. The 

 lateral plates eventually 

 bl.e unite ventrally, and their 



cavities coalesce to form 

 the body - cavity. The 

 parts derived from the 



FIG. 854. Embryoof a Ray with the medullary groove _ , 



closed except at the hind end. The notched em- mesoderm are tne System 

 bryonic part of the blastoderm has grown faster than f l 11T if QT-V rvmcplpc 



the rest and come to project over the surface of the ' Hdiy ^, 



yolk. bl. e. edge of blastoderm ; M. head ; neur. un- r]r> r ,Yiia flip i 

 enclosed part of the neuroccele. (After Sedgwick.) lfe ' L 



connective-tissue, the 



endoskeleton, the muscular 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 



M 2 



