No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 419 



ing from the ventral side of the body, immediately in front of 

 the mouth, towards the dorsal side of the embryo. This row of 

 cells has been called the prototroch, from its similarity to that 

 structure in the annelid trochophore. Owing to the marked 

 resemblance of this structure in the larvae of two such distinct 

 groups as mollusks and annelids, considerable interest is natu- 

 rally attached to a comparison of its cell origin in these forms. 



The cell origin of the prototroch was first determined in the 

 annelids by E. B. Wilson, who discovered that in Nereis the 

 four cells, \<f, \h^, etc., which he called the trochoblasts, gave 

 rise to this organ. Later, the origin of the prototroch in sev- 

 eral annelids has been carefully studied by Dr. A. D. Mead, with 

 the result that the cell origin of this organ was shown to be 

 identical in every case. In Amphitrite and Clymenella the cell 

 lineage of the prototroch was carried out in detail to a late stage, 

 but the other forms studied agree with these as regards the 

 protoblasts of this organ as far as their cleavage was observed. 

 In both Amphitrite and Clymenella the prototroch arises from 

 the four trochoblasts, which divide twice, forming sixteen cells, 

 and three of the cells of the second quartette in each of the 

 three quadrants, a, b, and c. Thus three out of the four cells 

 of the second quartette in each of these three quadrants go to 

 form the prototroch. Mead argues that in Nereis, also, the pro- 

 totroch arises in the same way, although Wilson's derivation of 

 this organ differs from Mead's as regards the fate of the upper 

 cells of the second quartette. Child's account of the formation 

 of the prototroch in Arenicola agrees, point for point, with 

 Mead's, and Mr. Treadwell's observations in Podarke indicate 

 a similar origin of the prototroch in that form (Child, '97; 

 Treadwell, '97). 



So far as known, the prototroch in the Mollusca arises in a 

 manner which is strikingly similar to its origin in the annelids. 

 Blochmann's derivation of the velum in Neritina from the pecul- 

 iar granulated tip cells of the lateral arms of the cross is doubt- 

 less incomplete, since these cells form only a part of this organ 

 in other mollusks, as in annelids. In Crepidula the velum at 

 first consists of a double row of cells, the dorsal ends of which 

 become "indistinguishable from the surrounding cells." "The 



