234 Edmund B. JFilson. 



equal, the small cell (id) being pure white, the larger showing 

 both upper and lower polar areas (Figs. 97-98). At the sec- 

 ond division (virtual fourth) the second somatoblast (2d) forms 

 from the lower polar area, while the micromere produces the 

 single trochoblast (id"), and the corresponding larger upper cell 

 id^ (Fig. 99). Beyond this the cleavage was not followed in 

 detail. It is noteworthy that in the divisions both of the halves 

 and the fourths the normal inequality of the cells is frequently 

 reduced, and this is frequently expressed by a reduction in the size 

 of the polar lobe, both in the CD-half and the D-fourth — in- 

 deed, I have seen only a few D-fourths In which the polar lobe 

 was of full normal size, and the first division of these cells is fre- 

 quently irregular and abnormal. This is doubtless due in part 

 to shock, perhaps also to the effect of the calcium-free water when 

 this is used. Nevertheless, I think it probable that the effect may 

 also be due in part to disturbances in the arrangement of the 

 cytoplasmic materials, which may possibly be interpreted as a 

 regulative phenomena. 



2. The partial cleavage in Patella. 



The cleavage of isolated halves or fourths in Patella is entirely 

 in agreement with the foregoing in being strictly partial in charac- 

 ter, but I wish especially to emphasize the fact that, precisely as 

 I showed in Cerebratulus, two general types exist, in one of which 

 the cells so shift as to produce a closed embryo from the begin- 

 ning, while in the other the blastula is at first widely open on one 

 side. The point is important because the effect of the displace- 

 ment in the closed type is to shift the primary trochoblast-groups 

 more or less widely, sometimes to opposite sides of the embryo, 

 while in the open type they remain in nearly their normal position. 

 Nature thus performs an experiment in the displacement of the 

 blastomeres closely similar to those carried out by Fischel on the 

 ctenophore-egg, and a corresponding result is produced that 

 clearly shows the differentiation of the cells to be independent of 

 their position in the embryo. 



