420 HOLMES. [Vol. XVI. 



median portion of the first row," says Conklin, "arises from the 

 cells which lie just beyond the ventral end of the apical plate. 

 These cells are in all probability i^''"=-''' and ib^-^-^-^-^ [inserting 

 the correction in Conklin's note, p. 204]. One of these cells 

 is shown dividing in Fig. 71. In Fig. 72 a transverse row is 

 formed, which is plainly the first row of velar cells." The por- 

 tion of the first velar row lateral to these six cells is evidently 

 derived from the anterior turret cells, la^ and ib^. The ante- 

 rior turret cells divide bilaterally in a nearly horizontal plane, 

 and these, with the median cells just mentioned, form a row of 

 cells, extending around the anterior half of the Qgg, connecting 

 the tip cells of the lateral arms. The tip cells of the lateral 

 arms divide, forming a transverse row of four cells, which forms 

 a further continuation of the first row of velar cells as far as the 

 undivided posterior trochoblasts and tip cells. 



Regarding the second row, Conklin says : "It is probable 

 that the mid-ventral portion of the second velar row, F^ is 

 derived from the cell which I have identified provisionally as 

 2<5•^■^ and which lies just beyond the median cells of the first 

 row (Figs. 56, 69, and 70). I have not been able to determine 

 whether any part of the second row arises by subdivision of the 

 cells of the first ; if not, this row may include a few cells of the 

 third quartette (3^''' and 3^'''', Fig. 56) at the points opposite 

 the anterior turrets. . . . Thus the preoral velum is composed 

 of a few cells of the first quartette, many of the second, and 

 possibly a few of the third." 



In Planorbis the cells composing the prototroch are few in 

 number and are arranged in a double row. The products of the 

 division of the tip cell of the anterior arm of the cross go to 

 form, as in Crepidula, a part of the upper row of cells. The 

 tip cell divides, as far as I can determine, but once, and the two 

 daughter-cells become pushed apart by the cell k^'"'''*, which 

 forms the median cell of the upper row. These cells extend 

 to the anterior trochoblasts on either side, but, in later stages, 

 they may sometimes be separated from them by cells which 

 wedge in from below. The anterior trochoblasts, which origi- 

 nally lay, the one above the other, became shifted by the ante- 

 rior rotation of the upper pole of the &gg, so that they come to 



