I20 CONK LIN. [Vol. XIII. 



cells 2a''', etc. (the tip cells), and does not show them in his 

 figures, I cannot determine whether the posterior arm in 

 Umbrella differs from the other arms, as it does in Crepidula. 

 In conclusion, the wonderful resemblance between Umbrella 

 and Crepidula in the history of the first quartette is shown to a 

 still greater extent in the history of the second. 



In both Nereis and Unio the cells of this quartette in quad- 

 rant D are unlike those of the gasteropods described. The 

 cell 2d is called by Wilson ('92) the "first somatoblast "; he has 

 followed it through a great many divisions, and has established 

 the fact that it gives rise to a large part of the ectoderm of the 

 trunk. This cell has a remarkably similar history in Unio 

 (Lillie, '95). It divides repeatedly, always in a bilateral way, 

 and apparently gives rise to parts of the body corresponding to 

 those which come from this cell in Nereis. 



In our present state of knowledge it is useless to attempt to 

 compare the bilateral cleavages of the first somatoblast in Ne- 

 reis and Unio with the spiral cleavages of the cell 2d in Neri- 

 tina. Umbrella, and Crepidula. A few of the earlier division 

 products may perhaps be compared ; e.g., in both Nereis and 

 Unio the first three cleavages give rise to similar cells, and at 

 least two of these cells, possibly three, may be compared with 

 the products of 2d in the gasteropods, as is indicated in the 

 following table : 



From its peculiar position both with regard to the somato- 

 blast and the cells which correspond to the molluscan cross, I 

 believe that x^ is the equivalent of the posterior tip cell in the 

 gasteropods. 



The division of the other members of the second quartette, 

 i.e., 2a, 2b, and 2c, can be compared with the divisions in the 

 gasteropods much more satisfactorily. In Nereis the divi- 

 sions of these cells are shown by the following table giving the 

 lineage of 2a : 



