380 HOLMES. [Vol. XVI. 



The twenty-four-cell stage, which is reached by these divi- 

 sions, marks a resting stage of considerable length in the devel- 

 opment of the Q^%. A cleavage cavity is formed at this period, 

 which may attain quite a large size. In the arrangement of the 

 cells the egg presents a perfect radial symmetry. The macro- 

 meres, or entomeres, as it is better now to call them, are some- 

 what larger than the cells above them, and their yellowish color 

 and greater opacity, due to the yolk they contain, render them 

 easily recognizable. Lying in the angles between the four 

 entomeres are the cells of the third quartette. These cells 

 are elongated in a meridional direction, the direction of their 

 next cleavage. Alternating with these cells, and hence lying 

 opposite the entomeres, are the lower cells of the second quar- 

 tette, 2c^, 2F, 2^^ 2^^ The four entomeres are, therefore, 

 surrounded by a circle of eight cells, of the second and third 

 quartettes. The four upper cells of the second quartette, when 

 viewed from the upper pole, lie a little to the left of the lower 

 ones, and do not come into contact with the cells of the ento- 

 derm. At the upper pole of the Q:%g there are eight cells, the 

 products of the cleavage of the first quartette of ectomeres ; 

 the four lower cells, the trochoblasts, alternate with the cells 

 of the second quartette, and lie opposite those of the third with 

 which they are in contact. The relation between these cells 

 corresponds essentially to the relation between the groups of 

 cells arising from them, even in a late stage of development. 

 The descendants of the four upper cells go entirely into the 

 formation of the cross, which will be described later ; below 

 these cells lie those of the second quartette, and below these 

 again the entomeres. Alternating with these groups, there 

 occur, in vertical arrangement, the trochoblasts, the third quar- 

 tette, and the angles between the entomeres. This arrangement 

 of the various quartettes, which is typical for molluscan spiral 

 cleavage, is a great aid in following their further history. 



There are a few points in which the cleavage of P. trivolvis 

 differs from Rabl's account of the cleavage of the form studied 

 by him, which it may be well to point out. Rabl's different 

 account of the cross furrows has already been discussed. The 

 passage from the twelve to the twenty-four cell stage takes 



