40 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
Since the minute cell de does not divide, we thus have produced 
sixty-three cells instead of the typical number, sixty-four. Such a stage 
does not, however, have an actual existence, sinoe some of the divisions 
belonging to the seventh cleavage have täken place before all these 
cleavages are finished. 
Figure 61 (Plate 7) shows the anterior surface of the egg at the end 
of the sixth cleavage, Figure 58 the posterior surface, Figure 60 the 
dorsal pole, and Figure 63 (Plate 8) the ventral pole. 
Sequence of the Sixth Cleavage. — The order in which the cells divide 
is, as I have already stated, now complieated by several factors. 
(1) The divisions of the first four quadrants of the egg (nt the third 
cleavage) were not synchronous, but followed in the order D, O, D A 
Other conditions remaining the same, — that is, with equal intervals 
between the ensuing cleavages, — the same order would obtain in the 
later stages. 
(2) As discussed on page 35, the sequence becomes modified during 
the fifth cleavage, so that the cells in any given quadrant divide in 
nearly or quite the order of size of the blastomeres, beginning with the 
largest. In the three quadrants A, B, and ©, the order is (the ven- 
tral cell in each case being considered number one) 4, 2, 1, 3, while in 
the quadrant D the order is 1, 2, 4, 3. This order would naturally 
reappear in the sixth cleavage, other conditions remaining the same. 
Both the above factors do influence the fifth cleavage, but with still 
further complications. The first factor appears in the fact, that in any 
given layer the general order of cleavage of the component cells is 
D, ©, B, A. 
The second factor is shown by considering the cleavage of a single 
quadrant, as D. The order of cleavage for the large left hand cells of 
this quadrant is as follows, naming the layers from ventral to dorsal: 
1, 2, 1$, — nearly or quite the same as at the last cleavage. 
jut a third factor appears in comparing the large left hand cells d. 
and , of the D quadrant with their small right band sister cells d&4 
and des (Plate 5, Fig. 37, Plate 6, Fig. 46, Plate 7, Figs. 54 and 57). 
The large cells of each pair divide first, though the age of the two, being 
sister colls, is exactly the same. 
Similar relations may be shown for the other quadrants, Two facts 
are worthy of particular notice. (I) The large cells a*" — c9" divide 
first of all the cells in the quadrants A, B, and €, — and long before 
the small cells aê — cs, which are of exactly the same age. (2) There 
are some variations which cannot be brought into relation with any of 
