No. 3.] PODARKE OBSCURA VERRILL. 469 



with the entodermal portion of 4d in Nereis ; while the defini- 

 tive mesoblast of Capitella is homologous merely with the 

 larval mesoblast of other forms. As already stated on p. 451, 

 I can hardly believe that these were normal cleavages. If 

 normal, they put an end to all questions of cell homologies, 

 complete or incomplete. 



The larval mesoblast is an organ whose exact origin is known 

 in a number of groups. It may arise from the second quar- 

 tette (Unio and Crepidula), from the third (Podarke, Physa, 

 Planorbis), or, if Eisig be correct, from the fourth (Capitella). 

 Its exact origin is probably not the same in any two of these 

 cases. It may arise asymmetrically, later becoming symmet- 

 rical (Unio and Podarke), or it may be symmetrical from the 

 beginning (Crepidula and Planorbis). Evidently no complete 

 cell homology can be demonstrated here, and we are given two 

 alternatives : either this larval mesoblast is not homologous 

 in all cases, or it arises from cells which are not completely 

 homologous. I prefer the latter assumption. 



Other organs of the embryo are the proctodaeum, stomo- 

 daeum, and supraoesophageal ganglia. While these are un- 

 doubtedly homologous organs, their precise cell origin is 

 different in the different cases. 



I have described these facts at some length, because it 

 seemed to me that the more we investigate the subject the 

 more difficulties do we find in the way of complete cell homolo- 

 gies, at the same time that marvelously close resemblances are 

 brought to light. I admit fully the justice of the position taken 

 by Conklin (see above, p. 465, first quotation), but I maintain that 

 with the single exception of the apical rosette (and the cell Xi .2 .^) 

 no organ of the annelid or mollusk has been traced back to 

 a similar cell in enough cases to establish cell homologies or 

 to justify Conklin's second statement (see above, p. 466, " We 

 therefore," etc.). 



This insistence upon a distinction between complete and 

 incomplete cell homology may seem a quibble, but I believe 

 the distinction is real, and that if we could prove complete cell 

 homology, we would be led logically to a very different theory of 

 development from what we must adopt if we regard homologies 



