No. 3-] EMBRYOLOGY OF UNIO COMPLANATA. 573 



to it, M (Fig. 4). This cell lies between X and D, and cor- 

 responds exactly to the second somatoblast or mesoderm 

 proteloblast of Nereis. Without further discussion I will 

 simply say, that it is the proteloblast of the mesoderm in Unio, 

 which is thus in every way identical with the mesoderm of 

 Annelids (Wilson), and of Gasteropods {inde Conklin on Cre- 

 pidula, and Erlanger^ on Bithynia). Shortly afterwards M 

 divides equally in a sagittal plane. 



If so far the minute agreement of the segmentation with 

 that of Nereis is surprising, I must confess that it seems little 

 less than astounding that the next divisions should agree. To 

 quote Wilson (Cell-lineage of Nereis, p. 41 1) : "At the second 

 division each of the primary mesoblasts buds forth a small cell 

 at the surface near its anterior margin." Exactly the same 

 thing occurs in Unio, though I cannot follow Wilson further 

 and say, "the budding of the mesoblasts is continued in the 

 same way for a considerable period." As a matter of fact I 

 have observed only this one superficial cleavage of the meso- 

 blasts, which are shortly afterwards, at the time of gastrulation, 

 taken within the segmentation cavity, where each forms a 

 "band" of cells. 



Let us return to the cell d- ox X. Here again we notice 

 an extremely close resemblance to the same cell in Nereis. 

 There is, however, one difference. In Nereis X buds off three 

 small cells before its division into two equal parts. In Unio 

 four such cells arise from X. These four divisions are as 

 follows : — x^ segmented off on the right side of the vegetative 

 pole, X- on the left side of the same pole, xl in the median line 

 towards the apical pole (and were a prototroch present in Unio, 

 such as is present in Nereis, it would cause a similar median 

 dorsal interruption of it), and x^ in the median plane of the 

 vegetative pole. (Not present in Nereis.) (Fig. 4.) X then 

 divides sagittally and equally, and each half as in Nereis soon 

 buds forth a small cell towards the vegetative pole. By con- 

 tinued division of the two large cells XI and Xr, a plate of 



^ Erlanger, Beitrage zur Entw. der Gasteropoden. Mitth. aus der Zool. Stat, 

 zu Neapel, Bd. X, Heft. 3. 



