296 



MESOBLAST AND NOTOCHORD. 



ventral parts of the primitive plates is formed the urinogenital 



system. 



That the above mode of origin of the mesoblast and noto- 



chord is to be regarded as a modifi- 

 cation of that observable in Am- 



phioxus seems probable from the 



following considerations : 



In the first place, the mesoblast is 



split off from the hypoblast not as a 



single mass but as a pair of distinct 



masses, comparable with the paired di- 



vcrticula in Amphioxus. Secondly, 



the body cavity, when it appears in 



the mesoblast p\a.tes,does not arise as a 



single cavity, but as a pair of cavities, 



one for each plate of mesoblast ; and 



these cavities remain permanentlydis- 



tinct in some parts of the body, and 



nowhere unite till a comparatively 



late period. Thirdly, the primitive 

 body cavity of the embryo is not 

 confined to the region in which a 

 body cavity exists in the adult, but 

 extends to the summit of tJie 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 mesoblast between the wall of the gut and 

 the body-wall ; but its extension to this part is quite intelligible, 

 on the hypothesis that it represents the cavities of two diver- 

 ticula of the alimentary tract, from the muscular walls of which 

 the voluntary muscular system has been derived ; and it may be 

 pointed out that the derivation of part of the muscular system 

 from what is apparently splanchnic mesoblast is easily explained 

 on the above hypothesis, but not, so far as I see, on any other. 



FIG. 185. SECTION THROUGH 

 THE TRUNK OF A SCYLLIUM 

 EMBRYO SLIGHTLY YOUNG KK 



THAN 28 F. 



sp.c. spinal canal; W. white 

 matter of spinal cord ; pr. poste- 

 rior nerve-roots ; ch. notochord ; 

 x. subnotochordal rod ; ao. aorta ; 

 vip. muscle-plate ; mp'. inner layer 

 of muscle-plate already converted 

 into muscles ; Vr. rudiment of 

 vertebral body ; si. segmental 

 tube ; sd. segmental duct ; sp.v. 

 spiral valve ; v. subintestinal vein ; 

 p.o. primitive generative cells. 



