ORIGIN OF THE GERMINAL LAYERS. 



293 



Coelenterata would lead us to expect that the mesoblast would be 

 derived partly from the epiblast and partly from the hypoblast. 



FIG. 211. SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE STAGES. (After Kowalevsky.) 



A. Section at gastrula stage. 



B. Section of a somewhat older embryo. 



C. Section through the anterior part of still older embryo. 



np. neural plate; nc. neural canal; me*, archenteron in A, and meseuteron in B 

 andC; ch. notochord; so. rnesoblastic somite. 



Such, however, is not for the most part the case, though more 

 complete investigations may shew that there are a greater number 

 of instances in which the mesoblast has a mixed origin than might 

 be supposed from the above summary. 



I have attempted to reduce the types of development of the 

 mesoblast to six; but owing to the nature of the case it is not 

 always easy to distinguish the first of these from the last four. Of 

 the six types the second will on most hands be admitted to be the 

 most remarkable. The formation of hollow outgrowths of the ar- 

 chenteron, the cavities of which give rise to the body cavity, can 

 only be explained on the supposition that the body cavity of the 

 types in which such outgrowths occur is derived from diverticula 

 cut off from the alimentary tract. The lining epithelium of the 

 diverticula the peritoneal epithelium is clearly part of the primi- 

 tive hypoblast, and this part of the mesoblast is clearly hypoblastic 



in origin. 



FIG. 212. SECTIONS THROUGH THE OVUM OF LEPTOPLANA TREJIELLARIS IN THREE 



STAGES OF DEVELOPMENT. (After HalleZ. ) 



ep. epiblast; m. mesoblast; hy. yolk-cells (hypoblast); bl. blastopore. 



Iii the case of the Chsetognatha (Sagitta), Brachiopoda, and 

 Amphioxus, the whole of the mesoblast originates from the walls of 

 the diverticula ; while in the Echinodermata the walls of the diver- 

 ticula only give rise to the vaso-peritoueal epithelium, the remainder 



