354 PROCEEDINGS OF THE ACADEMY OF [April, 
At a stage in which there are six cells 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 
leotropic in the right cell and dexiotropic in the left, while just the 
opposite is true of Fiona. Planorbis shows a group of second quartet 
eells 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 T’rochus, as in Fiona, show 
lack of alternation in the left cells of the series (2a71-2d”4), 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 leotropic 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 
