468 



TREADWELL. 



[Vol. XVII. 



The following table is not intended to be a 



investigations 



complete list of papers where the origin of this cell has been 



described, but contains the most recent. 



Nereis . 



Aricia 



Polymnia 



Amphitrite 



Arenicola 



Unio 



Umbrella 



Planorbis 



Crepidula 



Podarke 



Capitella 



\d contains 



Mesoderm and a little entoderm. 

 (( (1 (( 



" only (?). 



" only.i 



" and " secondary mesoderm " (entoderm?). 



" and a comparatively large amount of entoderm. 



" and a little entoderm. 



Larval mesoderm and ventral ectoderm. 



Here, leaving out of consideration, for the moment, the case 

 of Capitella, we must assume either that the cell 4d is not 

 completely homologous in Podarke and in Amphitrite or in 

 Unio and Crepidula, or we must assume that the mesoblast of 

 the one form is partly homologous with the entoderm of the 

 other ; and here again my preference is for the former assump- 

 tion. If Capitella be as described by Eisig, and we claim cell 

 homology for 4d, we are driven to the assumption that the 

 mesoblast of Nereis is homologous with the ventral plate of 

 Capitella, and the larval mesoblast of Capitella is homologous 



1 Professor Wilson has shown that it is not impossible that small cells budded 

 off from the primary mesoblasts in Amphitrite, Unio, Umbrella, Planorbis, and 

 Physa are really entoblasts and not mesoblasts. In Amphitrite, however, these 

 cells do not arise in the same position as in Aricia or Nereis, and they remain at 

 the anterior end of the mesoderm band ; their fate is therefore very different in 

 the two cases. Heymons is positive that in Umbrella these cells are mesoblast, 

 and in Planorbis they lie " in the cleavage cavity " (No. 1 5, a), hence have the 

 proper position at least for mesoderm. In a personal communication (1899) Dr. 

 Holmes assures me that there is no doubt of the mesoblastic fate of these cells. 

 For these reasons I have thought best to regard all these cells as mesoderm until 

 further study shall show that they are not. In Clymenella, also cited by Pro- 

 fessor Wilson, we have no evidence that the cells in question do not become 

 mesoderm, and the first division of the primary mesoblasts, as described by Mead, 

 may separate off an entodermal portion or it may be the first division toward the 

 formation of the mesoderm bands. At present we are unable to say which view 

 is correct, and the observations are of little use in this discussion. 



