JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 35 
The sequence of cleavage is the same as in previous divisions, but 
with some modifications. In any given layer of cells, the order is D, 
C, B, A; a repetition of the sequence established at the third cleavage. 
In any given quadrant, the order of cleavage varies with the relative 
size of the cells. In the quadrant D the order is (beginning with the 
ventral cell) 1, 2, 4, 3, and this is also the order of relative size of the 
cells, beginning with the largest. In the other quadrants the order of 
cleavage is 4, 2, 1, 3, and this again is the order of comparative size 
beginning with the largest, except that 2 and 1 are so nearly of a size 
that it is difficult to say from observation that either is the larger. 
The fifth cleavage is accompanied, as a result of the changes in form 
of the cells during karyokinesis, by a partial enclosure of the ventral 
cell of quadrant Y (d) by the other cells. 
The above account of the fifth cleavage is based upon an examination 
of twenty-five eges, taken from different individuals and showing differ- 
ent phases of the division; i.e. each egg contained more than fifteen 
and less than thirty-two cells. 
Sixth Cleavage. 
The first division belonging to the sixth cleavage, that of de, takes 
place coincidently with the last division of the fifth cleavage, that of a". 
There is thus no resting period between the two cleavages. Neverthe- 
less, there is a sufficiently well characterized stage of thirty-one or 
5.1 
thirty-two cells, just as the cleavage of de occurs, and it will be well 
to describe the egg in this condition as a basis for an account of the 
sixth cleavage. 
Figure 43 (Plate 5) shows the anterior surface just before this stage 
is attained ; Figure 47 (Plate G) shows nearly the same surface after the 
fifth cleavage is finished. The posterior surface is shown in Figures 46 
(Plate 6), 53, and 54 (Plate 7); the animal pole, in Figure 45 (Plate 
6). Figure 48 shows a sagittal section, while a transverse section of a 
stage just later (looking toward the animal pole) is given in Figure 52. 
The principal axis of the egg still coincides with its long axis, the 
animal pole lying at or near the micromere end, the vegetative pole at 
tlre macromere end (ig. 48). 
The egg now consists of (1) a single large cell, d**, embedded within 
the other cells and appearing on the surface at the ventral end only 
(Plate 5, Fig. 38), and (2) of thirty-one smaller cells, partly surround- 
ing the larger cell %%. One of these smaller cells, 4, is a minute 
vesicle embedded between the cells 45, 654 and q9* (Figs. 38 and 42). 
