1904.] NATURAL SCIENCES OF PHILADELPHIA. 349 
its component cells are neither among themselves distinguishable nor 
may they be definitely separated from the surrounding blastomeres. 
Of course, this is largely due to the multiplication of the trochoblasts 
and the similarity in size of most of the cells upon the upper surface of 
the egg, yet the leotropic twist given to the basal elements at their 
initial cleavage is largely responsible for that irregularity of contour 
which so early marks the outlines of the cross. The peripheral ends 
of the arms of the cross of Fiona become strongly twisted to the left, 
and as the structure becomes older the ends tend to bend around in 
that direction to a marked degree, greatly confusing their component 
cells with those arising by multiplication of the trochoblasts. Up to 
the stage shown in fig. 53 the cross has, with the exception of a slight 
tendency toward variation in the first division of the basals, been 
radially symmetrical, but at the next cleavage of the basals the cell 
of this series in the posterior arm divides so that its daughter cells 
he transverse to the longitudinal axis of this arm. In the anterior 
quadrant this division produces cells which lie radially, while in C 
quadrant the cleavage is leotropic, in A dexiotropic. 
The first indication of transverse splitting of the arms is thus seen 
to occur in the basal cell of the posterior quadrant. In Crepidula the 
reverse is the case, the anterior and lateral arms alone increasing in 
width, while the posterior later elongates by radial cleavages. In Fiona 
all the arms become longitudinally split at a later period. The inner 
and outer rosettes have not yet arisen in Crepidula when the splitting 
begins in the cells, la~b-c™, while in Fiona they are present and the 
egg contains many more cells, the basal cells of the anterior and lateral 
arms having again divided in such a manner that these arms are length- 
ened before increase in breadth occurs. The same is true of Planorbis. 
The early splitting of the arms of the cross in Crepidula is probably in 
part due, as Holmes suggests, to the fact that, through pressure, they 
have become much wider and tend to divide in a direction opposite to 
this elongation. It might also be suggested that the extreme breadth 
of the cross of Crepidula and the early transverse division of its anterior 
and lateral arms may be correlated with the presence of a large amount 
of yolk which must be covered by the ectoblast, while in the posterior 
region the extensive multiplication of the elements of the second quar- 
tet obviates the necessary broadening of the arm which reaches in that 
direction. 
The transverse cleavage of the anterior and lateral arms of the cross 
of Fiona occurs shortly after the initiation of a similar process in the 
posterior arm, but it has been found impossible to trace the lineage 
