JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRIOKII. 105 
46) are at opposite sides of the nucleus, the line joining them being 
parallel to the lateral axis of the embryo. 
In the egg seen in Figure 53 (Plate 7), the fifth division is finished 
in all the cells of the egg, (as shown by the study of the other quad- 
rants, which are not figured,) so that this is a later stage than that 
shown in Figures 43-46. The asters in des have taken an oblique 
position. 
In the egg of which Figures 48 (Plate 6) and 54 (Plate 7) are repre- 
sentations, the nuclei in the recently formed cells of quadrants A, 2, 
and O have enlarged, and the spindle is completely formed in d' (being 
dorso-ventral in position). Both of these facts show that the egg is older 
than the one shown in Figure 53. In this egg (Fig. 48) we find that a 
spindle is present in the entoderm cell d**, occupying nearly the position 
foreshadowed by the position of the asters in Figure. 42, and almost 
exactly the same position as the spindle at the foregoing division (Fig. 35). 
The egg seen in Figure 49 is still older, as shown by the presence 
of spindles in des and d^? (seen endwise), and the advanced condition of 
cleavage in d, Here de is just dividing, forming d"? and the second 
small cell, d“. 
Figure 50 is older than Figure 49, since d*5, dd, and d*5 have divided. 
In this egg we find that d*? has been separated into two cells, ds aud 
due, and there is a second small vesicle, die, in the position where it 
was seen in tho process of formation in Figure 49. 
The above is sufficient to illustrate the method of work ; the rest of 
the account might be analyzed in the same way. It is important to 
remember, however, that the deseription is not based merely upon the 
cases figured. Thus, for the processes just analyzed, more than thirty 
eggs, showing various phases of the changes occurring, were studied, 
while only eleven different eggs aro represented in the figures of these 
stages. 
The foregoing work was done in the winters of 1894-95 and 1895-96, 
in the Zoölogieal Laboratory of the Museum of Comparative Zoology at 
Harvard University. It gives me pleasure to acknowledge my grent 
indebtedness to the Director of the Laboratory, Professor E. L. Mark, 
for advice and assistance which have been of the greatest value to me 
throughout my work. 
