OPHIURA BREVISPINA 419 



tion is taken, there are eighty-nine dividing cells in the epithelial 

 wall and ten dividing mesenchyme cells. 



The configuration presented by the blastula of Ophiura there- 

 fore indicates that various forces are active in the differentiation 

 of its mesenchyme cells, among which is one, apparently the 

 dominating one, that is not intracellular in origin, but one which 

 actively involves the organism as a whole. It may be identified 

 with a process such as that referred to by Professor Whitman 

 in his well known essay entitled "The Inadequacy of the Cell 

 Theory," and is in harmony with the idea contained in the 

 'Tension Hypothesis' outlined by T. H. Morgan in his 'Regenera- 

 tion,' page 275. 



THE EPIGASTRIC COELOM AND THE ORGANIZATION OF 

 ECHINODERM EGGS 



A satisfactory account of the origin and differentiation of 

 the epigastric coelom of the larva of Ophiura could not be given 

 in my former paper for reasons stated therein. In my new 

 embryological material, however, there are larval stages in- 

 termediate between those formerly designated B and C, the 

 study of which enables me now to settle the question of the 

 origin of this part of the coelom and also to suggest an explana- 

 tion for other obscure points in the peculiar organization of 

 the egg and the larva. 



As formerly described and interpreted ('00, pages 88 and 90), 

 the internal organization of the larva of Ophiura, in the stages 

 in which various parts of the coelom are first morphologically 

 differentiated, is briefly as follows: 



A pair of small hollow pouches is constricted from the free 

 end of the archenteron, one pouch on the left and one on the 

 right, homologous with the anterior pair of enterocoeles developed 

 in the dipleurula larva of ordinary Ophiurids. A large unpaired 

 pouch is in the process of constriction from the middle, ventral 

 and left portion of the archenteron, which was at first inter- 

 preted to represent both members of the posterior pair of en- 



JOURXAL OF MORPHOLOGY, VOL. 27, NO. 2 



