354 PROCEEDINGS OF THE ACADEMY OF [April, 



At a stage in which there are six ceDs of the second quartet in each 

 quadrant in Crepidula these groups very closely resemble the similar 

 ones of Fiona. When there are four cells in each group in Crepidula 

 the larger middle pair divide and, as in Fiona, one of them shows lack of 

 alternation; but in Crepidula the direction of the cleavage is slightly 

 laeotropic in the right cell and dexiotropic in the left, while just the 

 opposite is true of Fiona. Planorbis shows a group of second quartet 

 cells in each quadrant, which may be said in this sinistral form to be 

 almost the mirrored image of the same cells of Fiona, though the tips 

 and the corresponding cells at the lower pole are somewhat larger in 

 Planorbis, which probably accounts for their earlier division in that 

 form. The large second quartet cells of Trochus, as in Fiona, show 

 lack of alternation in the left cells of the series (2a-^-2d^^), while the 

 right (2a^-2d^^) show regular alternation. The early cleavages in the 

 second quartet of Tethys (Viguier, 1898) closely parallel those of 

 the same series in Fiona. Viguier has mistaken the lower elements of 

 this quartet, 2a^^-2d^^, for members of the fourth, as Robert has pointed 

 out. Further note of the errors in this paper will not be taken here, 

 since they have been so thoroughly discussed by Robert. Heymons 

 (1893) for Umbrella shows the second quartet series up to a stage of 

 six cells in each quadrant, and here also similar conditions are found. 

 Carazzi (1900) figures the egg of Aplysia, where each quadrant contains 

 four second quartet cells, and here also is a marked similarity to the 

 other forms considered. The second quartet of Fiona maintains a 

 radial symmetry for a much longer period than Planorbis, this being 

 the result of similar cleavages in all four quadrants for a much later 

 period than in that Pulmonate. The same may be said of Umbrella 

 and Crepidula, and, as Holmes suggests, this phenomenon is probably 

 correlated with the earlier development and larger size of the head 

 vesicle of Planorbis than of the corresponding structure of Crepidula, 

 Umbrella or Fiona. 



The Third Quartet. 



Of the three quartets the third is the first to show evidences of 

 bilateral divisions. When the egg has cleaved into twenty-four 

 blastomeres this quartet has but one cell in each quadrant, and these 

 cells do not divide until after the second cleavage of the second quartet. 

 They then all divide in a laeotropic direction, but the resulting cells 

 are not of the same size in the different quadrants. 3a and 3b produce 

 cells of equal size, while 3c and 3d give rise to small cells in the direction 

 of the vegetative pole with very large ones above, thus forming an 



