DEVELOPMENT OF VERTEBRATES. 1 29 



others, in which the body cavity arises as an outgrowth of the 

 alimentary canal and the somatopleure and splanchnopleure are 

 formed from that outgrowth, it is clear without further remark 

 that the mesoblast is derived from the hypoblast. For the 

 Echinoderms in which the water-vascular system and muscular 

 system arise as a solid outgrowth of the wall of the alimentary 

 canal there can also be no question as to the derivation of the 

 mesoblast from the hypoblast. 



Amongst other worms, in addition to Sagitta, the investi- 

 gations of Kowalevsky seem to shew that in Lumbricus the 

 mesoblast is derived from the hypoblast. 



Amongst Crustaceans, Bobretsky's 1 observations on Oniscus 

 (ZeitscJirift fiir wiss. Zoologie, 1874) lead to the same con- 

 clusion. 



In Insects Kowalevsky 's observations lead to the conclusion 

 that mesoblast and hypoblast arise from a common mass of 

 cells; Ulianin's observations bring out the same result for the 

 abnormal Poduridae, and Metschnikoff' s observations shew that 

 this also holds for Myriapods. 



In Molluscs the point is not so clear. 



In Tunicates, even if we are not to include them amongst 

 vertebrates 2 , the derivation of mesoblast from hypoblast is with- 

 out doubt. 



Without going further into details it is quite clear that the 

 derivation of the mesoblast from the hypoblast is very general 

 amongst invertebrates. 



It will hardly be disputed that primitively the muscular 

 system of the body wall could not have been derived from the 

 layer of cells which lines the alimentary canal. We see indeed 

 in Hydra and the Hydrozoa that in its primitive differentia- 

 tion, as could have been anticipated beforehand, the muscular 

 system of the body is derived from the epiblast cells. What, 

 then, is the explanation of the widespread derivation of the 

 mesoblast, including the muscular system of the body, from the 

 hypoblast ? 



1 He says, p. 182 : " Bevor aber die Hiilfte der Eioberflache von den Embryonal- 

 zellen bedeckt 1st, kommt die erste gemeinsame Anlage des mittleren und unteren 

 Keimblattes zum Vorschein." 



2 Anton Dohrn, Der Ur sprung des Wirbelthieres. Leipzig, 1875. 



B. 9 



