246 



ME SO BLAST AND NOTOCHORD. 



not confined to the region in which a 

 body cavity exists in the adult, hut ex- 

 tends to the summit of the muscle-plates, 

 at first separating parts which become 

 completely fused in the adult to form 

 the great lateral muscles of the body. 



It is difficult to understand how the 

 body cavity could thus extend into the 

 muscle-plates on the supposition that it 

 represents a primitive split in the meso- 

 blast between the wall of the gut and 

 the body- wall ; but its extension to this 

 part is quite intelligible, on the hypo- 

 thesis that it represents the cavities of 

 two diverticula of the alimentary tract, 

 from the muscular walls of which the 

 voluntary muscular system has been de- 

 rived ; and it may be pointed out that 

 the derivation of part of the muscular 

 system from what is apparently splanch- 

 nic mesoblast is easily explained on 

 the above hypothesis, but not, so far as 

 I see, on any other. 



Such are the main features, presented 

 by the mesoblast in Elasmobranchii, 

 which favour the view of its having 

 originally formed the walls of the ali- 

 mentary diverticula. Against this view 

 of its nature are the facts (1) of the 

 mesoblast plates being at first solid, and 

 (2) of the body cavity as a con- 

 sequence of this never commu- 

 nicating with the alimentary 

 canal. These points, in view of 

 our knowledge of embryological 

 modifications, cannot be regard- 

 ed as great difficulties in my 

 hypothesis. We have many ex- 

 amples of organs, which, though 

 in most cases arising as involu- 

 tions, yet appear in other cases 

 as solid ingrowths. Such ex- 

 amples are affijrded by the optic 

 vesicle, auditory vesicle, and 

 probably also by the central ner- 

 vous system of Osseous Fishes. 

 In most Vertebrates these or- 

 ga.ns are formed as hollow in- 



sd" 



Fig. 185. Section through 

 the trunk of a scyllium embryo 

 slightly younger than 28 f. 



sp.c. spinal canal; W. white 

 matter of spinal cord; pr. poste- 

 rior nerve-roots; ch. notochord; 

 X. sub-notochordal rod; ao. aorta; 

 mp. muscle-plate ; mp'. inner layer 

 of muscle-plate already converted 

 into muscles ; Vr. rudiment of 

 vertebral body; st. segmental 

 tube; sd. segmental duct; sp.v. 

 spiral valve ; v. subintestinul vein ; 

 p.o. primitive generative cells. 



Fig. 186. Horizontal section through 



THE trunk of an EMBRYO OF ScYLLIUM CON- 

 SIDERABLY YOUNGER THAN 28 F. 



The section is taken at the level of the 

 notochord, and shews the separation of the 

 cells to form the vertebral bodies from the 

 muscle-plates. 



c/(. notochord; ^p. epiblast; Fr. rudiment 

 of vertebral body; mp. muscle-plate; mp'. 

 portion of muscle-plate already differentiated 

 into longitudinal muscles. 



