460 TREADWELL. [Vol. XVII. 



apparently has the same origin and fate in Nereis, Amphitrite, 

 Unio, and Clymenella. 



The most complete homology, according to Mead, is shown 

 in the origin of the mesoderm. This always arises leiotrop- 

 ically at the ideal 64-cell stage, and is one of the fourth genera- 

 tion of micromeres given off from the cell D. A serious 

 discrepancy here and a strong argument against cell homology 

 was the origin of the mesoblast in Discocoelis, where, accord- 

 ing to Lang (No. 19), although the general form of the cleav- 

 age is the same as the annelid, the mesoderm arises not from 

 4d, but from the second and third group of micromeres, the 

 cell 4d forming a part of the entoderm. Lang's figures, how- 

 ever, show a bilateral division of 4d very different from the 

 division of the other members of this quartette, and Mead has 

 suggested that possibly there was an error in the interpreta- 

 tion, and that mesoblast may arise from this cell in the platode 

 as well as in the annelid. 



In his " Embryology of Crepidula," published a little earlier 

 than Mead's paper, and in a subsequent lecture (No. 5, a and ^), 

 Conklin has extended the comparison between annelids and 

 mollusks still farther. After mentioning the similarities above 

 noted, he says: "To this list of resemblances between the 

 annelid and the mollusk, which I can confirm in the case of the 

 gasteropod, I have been able to add the following : The rosette 

 series of the gasteropod is exactly like the cross of the annelid 

 in origin, position, and probably in destiny. The intermediate 

 girdle cells of the annelid are like the cross of the gasteropod 

 in origin, position, and destiny (at least in part). The differ- 

 ences, therefore, between the annelidan and molluscan cross 

 which Wilson emphasizes are not real ones. The trochoblasts 

 of the annelids and gasteropods are precisely similar in origin 

 and destiny, at least in part.'" (The italics are mine. See p. 466.) 

 " In some annelids (Amphitrite, Clymenella, Arenicola) the 

 prototroch is completed by cells of the same origin as in Crepi- 

 dula and Neritina. The differences which Wilson points out 

 between these two structures do not therefore exist. In both 

 annelids and mollusks the prototroch lies at the boundary be- 

 tween the first quartette on one side and the second and third 



