JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKI. 29 
which are of the greatest importance, have been taking place, partly as 
a consequence of these cleavages. As the constriction of the cyto- 
plasm in dë? and dë? occurs, these cells show in a most pronounced way 
the tendency to become of a rounded form. The inner portions of the 
cell are withdrawn still more from the centre of the egg, until the an- 
tero-posterior measurements are no greater than their dorso-ventral 
dimensions, This is shown in the section, Figure 38. In this egg, a 
surface view of which is given in Figure 37, the cleavage of dë? is 
finished, and the products, des and de, have already passed into the 
“resting stage," so that they take whatever form is impressed upon 
them by the surroundings. But 45.8 is just dividing into de and ders, 
and the form shown in section by d95 in Figure 38, as compared with 
the form of des in Figure 32, shows the change which I have been 
describing. 
At the same time the cells of the other quadrants, A, B. and C, are 
entering upon the stages preparatory to karyokinetio division. As a 
first step they also retract their deeper parts and bring their protoplasm 
into a more compact mass, as shown by a comparison of quadrant B 
in Figure 34 (Plate 4) with the similar quadrant, A, in Figure 38 
(Plate 5). 
As a consequence of this withdrawal of material from the inner parts 
of the egg, the large ventral cell de, which has now passed into the 
resting stage, moves inward to occupy the space which would otherwise 
be vacant, — being forced to do so, of course, by tho greater dorso-ventral 
extension of all the other cells. The result is shown in Figure 38. 
This partial enclosure of d' by the other cells is of course a stage in 
the process of gastrulation. 
Before the cleavage of all the cells of quadrant D is finished, the 
karyokinetie processes have begun in the other three quadrants. (See 
Plate 5, Figs. 39-42.) The first cells to show the characteristie nu- 
clear phenomena are those of the fourth or dorsal layer, % — . As 
previously stated, the asters at first take up such a position in these 
cells as would lead, if unchanged, to a meridional cleavage. But in these 
three cells, as in d55, there is a revolution of the asters and nuclei, 
resulting in a dorso-ventral position of the spindles. At any given period 
the three cells are not in exactly the same phase of division, though very 
nearly so; the order, beginning with the most advanced, is c4, 054, ad, 
The sequence is thus the same as in previous cleavages. The division is 
equatorial and the dorsal product is the smaller, as in the cleavage of 
the corresponding cell (dss) of the left posterior quadrant. In tho three 
VOL. XXX. — NO. 1. 3 
