JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKIL 53 
entirely closed, as shown in Figure 73.) The blastopore notch lies, not 
directly at the macromere end of the egg, but at some distance on the 
posterior side of it. 
As the cells of the anterior lip of the blastopore are turned inward, 
some of them begin to divide. Spindles in these cells are shown in 
Plate 9, Figs. 75 and 77. The form and position of the cells have 
changed so much at this time that it is impossible to determine with 
certainty whether the cleavage should be considered equatorial or merid- 
ional. In the cases figured the spindles are nearly or quite transverse, 
so that in some of the cells the division is meridional. 
Dracram VI. 
Diagram of quadrant D at a time when 
mest of the cells have passed into the 
ninth generation. All cells in the ninth 
generation are bounded by continuous 
lines, and are designated by the second 
exponent belonging to the cell, the first 
exponent being in each case 9. In the 
other cells both exponents are given. 
The arrows connect cells of common 
origin, and show the direction of the 
spindle at the last division. 
R signifies right; L, left; D, dorsal; 
V, ventral, 
The cleavage of the ectoderm has now been traced to the eighth gen- 
eration in all parts of the egg, and in the greater part of the quadrant 
D to the ninth generation. 
With this ends the account of the cleavage of the ectodermal cells. 
The small size and the great displacements of the blastomeres, espe- 
cially in the regions of the blastopore and the animal pole, render it 
impossible to determine with certainty their identity in later stages, 
and the real direction of eleavage is masked by erowding and deforma- 
tion of the cells. It would perhaps be of little interest in connection 
with the laws of cleavage to carry the study further, as it is scarcely to 
be presumed that the later divisions would exhibit any phenomena dif- 
