COMPARISON OF THE GERMINAL LAYERS. 



295 



The undivided ventral portion gives rise to the general 



IHJ> 



FIG. 183. THREE SECTIONS OF A PRISTIURUS EMBRYO SLIGHTLY OLDER THAN 



FIG. 18 B. 



The sections shew the development of the notochord. 



Ch. notochord; CH. developing notochord; mg. medullary groove; Ip. lateral 

 plate of mesoblast ; ep. epiblast ; hy. hypoblast. 



somatic and splanchnic 

 mesoblast (fig. 185), 

 and the cavity between 

 its two layers consti- 

 tutes 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 oblite- 

 rated. Dorsally they 

 are separated by the 

 mesentery. From the 

 mesoblast at the junc- 

 tion of the dorsal and 



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 differentiation of 

 muscles ; ch. notochord ; jr. subnotochordal rod 

 arising as an outgrowth of the dorsal wall of the 

 alimentary tract ; al. alimentary tract. 



