THEORY OF THE EMBRYONIC STAGE OF ONTOGENY. 335 



Russo,^ but neither the figures nor the methods of this author 

 are calculated to inspire much confidence. These earlier stages 

 are very difficult to obtain, but I have strong reason to suspect 

 from those which I have seen, that the earlier development 

 follows the ordinary Echinoderm type. 



Having thus rapidly reviewed the principal disturbing 

 factors in embryonic development, we can employ our know- 

 ledge in attacking one of the most vexed questions in mor- 

 phology, viz., the significance of the mesoderm and its contained 

 cavity the ccelom. 



Is the former to be regarded as a differentiated portion of 

 the gut-wall, and the latter as a portion of the enteric cavity, 

 or is the coelom to be regarded as a mere enlargement of the 

 cavity of the gonad as Hatschek^ has suggested ? 



In all Annelids and all Mollusca (Paludina and Cephalopods 

 excepted) the mesoderm first appears as two symmetrically 

 situated large cells — the primary mesoblasts. In Paludina, 

 Echinodermata, Sagitta, Brachipoda, and Amphioxus, it arises 

 as one or more pouches of the gut. Now, leaving out of 

 account Anthropods, Vertebrates and Cephalopods, where the 

 development has been complicated by the enormous amount 

 of yolk present, we find that of the other groups the Echino- 

 derms have by far the most prolonged larval development. 

 They are unique amongst the Coelomata in the fact that the 

 blastosphere is a free-swimming larva, and that consequently the 

 development of both endoderm and mesoderm takes place during 

 their free-swimming life. Here, then, we may on our hypothesis 

 expect to find ancestral structure preserved, and here we find that 

 the coelom is developmentally a part of the archenteric cavity. 



No Annelid or Molluscan larvse commence free life so early ; 

 most of them may be ruled out at once for the purposes of 

 this comparison, since the disturbing presence of yolk shows 

 itself plainly in the fact that the endoderm is represented by 

 a few large spheres, and the production of a pouch has become 



^ Achille Rosso, " Etnbryologia d' Amphiura squaniata," ' Rendiconti 

 della Societa Reale di Napoli,' tome vii (2nd series). 

 =" ' Lehrbuch der Zoologie,' 1891, B. Hatschek. 



