238 Edmund B. JVihon. 



however, in the comparison of the two halves from the same egg, 

 in which Patella differs from Dentalium, namely, that both halves 

 develop an apical organ; and while this has not been directly 

 proved for the four quarters, the fact, described above, that any 

 isolated micromere of the first quartet may develop an apical or- 

 gan leaves practically no doubt that the same is true for the quar- 

 ters. The basis of the apical organ in Patella must, therefore, 

 be symmetrically divided by the first two cleavages, while it re- 

 mains undivided in Dentalium, remaining as a whole in the D- 

 quadrant. This is possibly correlated with the fact that the apical 

 rosette, formed at the fifth cleavage of Patella nnd Trochiis, fails 

 to appear in Dentalium, where the i^-^ cells are as large as their 

 sister-cells i^". 



In Dentalium, as described in my first paper, the larvae in- 

 variably close sooner or later, and the prototroch, in most if not 

 all cases, closes also to form a complete belt encircling the body. 

 In Patella, however, this is not always the case; and frequently 

 the >4-larv2e of 24 hours show the prototroch as an area of char- 

 acteristic trochoblasts extending around one side only, terminating 

 abruptly to leave a space occupied by much smaller non-ciliated 

 cells. ^ (Figs. 117, 118). In other half larvae the prototroch ap- 

 pears as a complete belt, in still others as a more or less irregular 

 or interrupted structure. 



An examination of the earlier ciliate^l stages, combined with 

 the results obtained with isolated trochoblasts, gives the obvious 

 explanation of these differences. In those of the earlier larva? 

 (8-10 hours) that are still open on one side (and hence must 

 have been derived from the open type of cleavage) two adjoin- 

 ing groups of trochoblasts are found on one side, leaving a space 

 on the opposite side free from trochoblasts (Fig. 114). In the 

 closed embryos, on the other hand, two corresponding trocho- 

 blast groups are formed on opposite sides of the embryo, with 

 only rather narrow gaps between them (Figs. 11 3- 11 5- 116). 

 Both these types may be represented in twins from the same egg, 

 a case which I am fortunately able to show by Figs. 113 and 114 



iln agreement with Crampton's observation that the i^-larvse of Ilyanassa 

 form "a partial circle of cilia" (96, p. 9). 



