ORIGIN OF THE GERMINAL LAYERS. 355 



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 archenteron, 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 primitive hypoblast, and this part of the 

 mesoblast is clearly hypoblastic in origin. 



FIG. 112. SECTIONS THROUGH THE OVUM OF LEPTOPLANA TREMELLARIS IN 



THREE STAGES OF DEVELOPMENT. (After Hallez.) 

 cp. epiblast ; ;;/. mesoblast; hy. yolk-cells (hypoblast); bl. blastopore. 



In the case of the Chaetognatha (Sagitta), Brachiopoda, and 

 Amphioxus, the whole of the mesoblast originates from the walls 

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

 diverticula only give rise to the vaso-peritoneal epithelium, the 

 remainder of the mesoblast being derived from amoeboid cells 

 which spring from the walls of the archenteron before the origin 

 of the vaso-peritoneal outgrowths (figs. 199 and 210). 



Reserving for the moment the question as to what conclu- 

 sions can be deduced from the above facts as to the origin of the 

 mesoblast, it is important to determine how far the facts of 

 embryology warrant us in supposing that in the whole of the 

 triploblastic forms the body cavity originated from the alimentary 

 diverticula. There can be but little doubt that the mode of 

 origin of the mesoblast in many Vertebrata, as two solid plates 

 split off from the hypoblast, in which a cavity is secondarily 

 developed, is an abbreviation of the process observable in 

 Amphioxus ; but this process approaches in some forms of 



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