No. 395.] REVERSAL OF CLEAVAGE IN ANCYLUS. 873 
to get the egg out of the capsule without injury, and the chance 
of losing it in the process of fixation, staining, etc., before it 
is safely mounted, is very considerable, so that no attempt was 
made to carry the cleavage very far. At the first cleavage the 
egg divides into two equal cells, which round off in the usual 
manner and then become more closely approximated shortly 
before the division into four cells. The eight-cell stage is 
formed by the unequal division of the four cells, each cell 
giving off a smaller clear cell at 4 
the animal pole. This cleavage 
is an oblique one, the daughter 
cells lying in the angles between 
the larger cells. The smaller cells 
do not lie exactly midway between 
the larger ones, but more nearly 
over the cells from which they M 
arose. The next cleavage takes -Y 
place in the four larger cells or Ke core i 
macromeres, and, as indicated in animal pole. 
Fig. 2, the direction of cleavage 
is shown to be in a right-handed 
spiral by the inclination of the 
spindles — just the reverse of the 
corresponding cleavage in the un- 
reversed type. The cleavage of 
the egg was not followed beyond 
this point, but it is very probable 
ll at n fro th, 
eee Fic. 2.— The same egg seen obliquely, 
that the subsequent divisions would showing the dexiotropic inclination of 
the spindles in the di en macromeres. 
show the same reversal. In Physa, Was cit Chae 
according to Crampton, every di- 
vision as far as the twenty-eight-cell stage is the reverse of the 
corresponding division in the closely allied genus Lymnea, and 
in Planorbis, in which I have followed the cleavage in detail 
to beyond the 150-cell stage, the reversed type of cleavage 
holds, except in one or two divisions, as far as the cleavage is 
of a determinate spiral character. 
It is one of the striking features of the cleavage of many 
mollusks and annelids that certain cells of the upper -side of 
