134 BIOLOGICAL LECTURES. 



become functional until a very late period of cleavage, so that 

 it is almost impossible to trace its cell-origin accurately. 



Blochmann, in his work on Neritina, was able to trace the 

 descendants of two cells in the early cleavage into the velum. 

 They could be traced the more readily because of a peculiar 

 texture which distinguished them from the surrounding cells. 

 These two cells in Neritina correspond precisely to the two 

 lateral secondary trochoblasts of Amphitrite and the other 

 annelids and Crepidula (the two lateral cells shaded with lines 

 in Fig. 9). The primary trochoblasts of Neritina are also dif- 

 ferent from the surrounding cells. They are very minute, as 

 in Scolecolepis, and were never observed by Blochmann to 

 divide. As far as observations go, therefore, Neritina falls in 

 line with the other forms which we have studied. 



Conklin has called attention to the altogether characteristic 

 appearance of the primary trochoblast, not only in Neritina 

 and in four species of Crepidula which he studied, but in 

 Umbrella (Heymons), Urosalpynx, and Fulgar. In all it is 

 conspicuous for its small size, and does not divide for a 

 comparatively long time. 



It will appear from Holmes's preliminary paper that the 

 origin of the velum in Planorbis bears a remarkable resem- 

 blance to that of the annelids. Planorbis resembles AmpJiitrite 

 even more than it does Crepidula in this respect, viz., the primary 

 trochoblasts are large when formed, and the whole prototroch 

 becomes functional at an early period. With regard to the 

 cell-origin of the prototroch Holmes says: "The prototroch is 

 formed from the [primary] trochoblasts previously mentioned, 

 and the uppermost cells of the second quartette which form the 

 tips of the arms of the cross [secondary trochoblasts, q, r, s, t, 

 in the three quadrants]. Possibly other cells of the second 

 quartette may take part in this formation ; it is certain that the 

 third quartette has no share in the process." 



I am greatly indebted to Mr. Harold Heath for a very com- 

 plete account of the origin of the prototroch in Ischnochiton, 

 and for his courtesy in allowing me to use his discoveries 

 in this paper. The four primary trochoblasts are formed at 

 the i6-cell stage, exactly as in Amphitrite, etc., etc. (Fig. 7, 



