348 PROCEEDINGS OF THE ACADEMY OF [April, 
ing in eight cells of equal size and occurring at relatively the same time 
in all four quadrants. 
With the completion of the above-described divisions the large num- 
ber of cells of similar size at the upper pole of the egg makes their exact 
lineage difficult to follow, so that it is desirable to make here some com- 
parisons with the structure and development of the cross and trocho- 
blasts in other forms, and to bring together the results already obtained 
before proceeding to more uncertain ground. In formation the cross 
of Fiona arises in the same manner as in Umbrella and Planorbis, by 
the completion of the tip cells before the basals; and in this it differs 
from Neritina and Crepidula, where the tip arises shortly after division 
has occurred to form the four basal cells. In Trochus the tips are 
relatively late in appearing, as the basals have completed their cleavage 
before these cells arise. At the first cleavage of the basals another 
striking similarity to Umbrella is found, for in this Opisthobranch the 
cleavage is leotropic, while in Crepidula and Neritina it is dexiotropic, 
thus breaking the law of alternating cleavages; and likewise in Planorbis 
with reversed type the division is leotropic and non-alternating with 
the preceding. Trochus shows an extremely marked leotropic division 
of these cells, so much so, in fact, that the resulting cells le almost 
transversely. In Fiona the anterior and posterior basals are distinctly 
leotropic in origin and so regularly alternating, while considerable varia- 
tion is found in the lateral arms, a radial type often occurring with 1c”, 
sometimes showing a decided dexiotropic direction of spindle. It 
would appear from this variation in the lateral arms that Fiona shows 
tendencies toward bilaterality in the first quartet at this time, and such 
a condition would be in harmony with the bilateral cleavages of the 
third quartet cells, 3c and 3d', occurring just previously. However, 
the radial symmetry of the cross as a whole appears not to be dis- 
turbed appreciably, so that though these variations may show either a 
tendency toward bilaterality or toward entire reversal in all quadrants, 
as is found in Neritina, Crepidula and Planorbis, this influence has not 
as yet become sufficiently marked to affect the radial symmetry of 
the upper pole of the egg to any appreciable degree. In discussing the 
lack of alternation of these cleavages in Crepidula as opposed to alter- 
nation in Umbrella, Conklin suggests “‘upon this difference the future 
recognizability of the cross in the last-named cases (Crepidula and 
Neritina) depends”. In Umbrella the leotropic division of the basals 
is much more marked than in Fiona, but even in the latter case Conk- 
lin’s prediction is in part, at least, fulfilled, as the cross of Fiona, after 
a slightly older stage than thus far described, becomes so irregular that 
