COMPARISON OF THE GERMINAL LAYERS. 



245 



and splanchnic mesoblast 

 (fig. 185), and the cavity 

 between its two layers con- 

 stitutes the body cavity. 

 The originally separate 

 halves of the body cavity 

 eventually meet and unite 

 in the ventral median line 

 throughout the greater 

 part of the body, though 

 in the tail they remain 

 distinct and are finally ob- 

 literated. Dorsally they 

 are separated by the me- 

 sentery. From the meso- 

 blast at the junction of the 

 dorsal and ventral parts 

 of the primitive plates is 

 formed the urinogenital 

 system. 



That the above mode 

 of origin of the mesoblast 

 and notochord is to be re- 

 garded as a modification 

 of that observable in Am- 

 phioxus seems probable 

 from the following con- 

 siderations : — 



In the first place, the 

 mesoblast is split off from 

 the hypoblast not as a 

 single mass but as a pair 

 of distinct masses, com- 

 parable with the paired 

 diverticula in Amphioxus. 

 Secondly, the body cavity, 

 when it appears in the 

 mesoblast plates, does not 

 arise as a single cavity, but 

 as a pair of cavities, one 

 for each plate of mesoblast ; 

 and these cavities remain 

 permanently distinct in 

 some parts of the body, 

 and nowhere unite till a 

 comparatively late period. 

 Thirdly, the primitive body 

 cavity of the embryo is 



Fig. 183. Three sections of a Peistiubus 

 embryo slightly older than fig. 28 b. 



The sections shew the development of the noto- 

 chord. 



Gh. notochord; C/t'. developing notochord ; mg. 

 medullary groove; Ip. lateral plate of mesoblast; 

 ep. epiblast; hy. hypoblast. 



m^ 



Fig. 184. Transverse section through the 



TAIL-REGION OF A PriSTIURUS EMBRYO OF THE SAME 

 AGE AS FIG. 28 E. 



df. dorsal fin ; sp.c. spinal cord ; pp. body- 

 cavity ; sp. splanchnic layer of mesoblast ; so. 

 somatic layer of mesoblast ; mp'. commencing dif- 

 ferentiation of muscles ; ch. notochord ; x. sub- 

 notochordal rod arising as an outgrowth of the 

 dorsal wall of the alimentary tract ; al. alimentary 

 tract. 



