50 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
in the short axes of the cells, as shown in Figure 75 (Plate 9). Just 
before division the cells elongate until the axis in which the spindle lies 
is longest, as shown in the cell 0^? of the figure last mentioned. 
The cells e e, near the animal pole, cleave meridionally. The 
spindle in a’, and the cells 6° and 05.2, 09? and 6*9 are shown in 
Figure 72 (Plate 9). The minute cells d — 6739, like die, do not 
cleave further. The arrangement at the animal pole is now very 
irregular, and owing to this fact and the minuteness of the cells pro- 
duced at the last cleavages it is very diffioult to be certain of the exact 
origin of any given cell, though the origin of the growp cannot of 
course be doubtful. 
We have now accounted for all the cells of these quadrants except 
2 7.2 
, 
the four ventral cells of each quadrant, a! e, at? — 672, a" — , and 
Aus u. These correspond in origin to the entoderm cells of quadrant 
D, and they do not cleave further until they are partly or entirely 
enclosed within the embryo, as will be shown later. 
A diagram of that part of any one of the quadrants A, B, and C 
corresponding to the ectodermal part of quadrant D, and showing the 
conditions at the end of the seventh cleavage, is given on page 48 (Dia- 
gram V.). A comparison of this diagram with that for the correspond- 
ing stage of the quadrant D (Diagram IV. on the same page) shows 
that the direction of the cleavage planes, and hence of the spindles, is the 
same throughout in all cells of corresponding position, though there are 
many differences as to the equality or inequality of the cleavage products. 
The general facts which may be deduced from the foregoing study of 
the seventh cleavage in tho ectodermal parts of the egg are similar to 
those drawn from a study of the sixth cleavage, page 41. 
(1) Every cell of any quadrant divides with its spindle in the same 
direction as the corresponding cell of any other quadrant. 
(2) All the cells in any layer cleave with spindles in the same direc- 
tion (in spite of great differences in the form of the cells.) 
(3) No general law can be deduced as to the equality or inequality of 
the divisions. 
(4) There is a tendency for the largest cells to cleave fastest. Certain 
very small cells (at the dorsal pole) do not cleave at all. 
Other Changes during the Seventh Cleavage. — In almost every cleav- 
age which has taken place, whenever the division was equatorial, — the 
spindles taking a dorso-ventral position, — it will have been noticed that 
the axis in which the spindle was formed was the short axis of the cell. 
On the other hand, the cells in which meridional cleavages have taken 
