88 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
tion. A spindle parallel with the chief axis would be in harmony with 
Groom’s view that the first cleavage furrow is perpendicular to that 
axis. Numerous transparent preparations of entire eggs have convinced 
me that such is never the case. 
In the review of literature on maturation and fertilization I have 
already referred to Groom’s mistake in identifying the pronuclei as the 
daughter-nuclei of the segmentation nucleus. He speaks (p. 145) of two 
nuclei seen in “the first blastomere” (cell ab? of this paper). One of 
the two nuclei which he regards as the daughter-nuclei of the segmenta- 
tion nucleus remains as the nucleus of “ the first blastomere,” the other 
passes into the “ yolk hemisphere ” (yolk-cell ed? in this account) just 
before the cell-plate is formed. This is certainly erroneous, and is ap- 
parently the result of his interpretation of the transverse furrow accom- 
panying maturation as the cleavage furrow. In Groom’s Figure 8 two 
distinct nuclei are represented in the “protoplasmic” part of the egg, 
which he considered ‘‘the first blastomere.” It is evident from my 
figures that the daughter-nuclei of the segmentation nucleus could not 
normally get into such a position; but the pronuclei are often seen on 
one side of the constriction during maturation phases (see my Figure 
18). I interpret Groom’s Figure 8 as representing the pre-cleavage 
stage corresponding to my Figures 3 and 18, and the lower half of the 
egg as the yolk-lobe, not the yolk-cell cd”. I have already stated that, 
unless eggs are kept under continuous observation, it is easy to confuse 
this stage with the two-cell stage, when only living eggs are examined. 
My series of figures shows that no such interpretation as that above 
quoted fits the facts. There are two nuclei (pronuclei) in the proto- 
plasmic hemisphere during the later maturation phases (Figs. 18, 20) ; 
but in the “ first blastomere ” (cell ab? in my Figs. 26, 27) there are never 
two, one of which is destined to pass into the yolk. Groom’s description 
of the “ yolk” (cell ed?) as at first without a nucleus, but receiving one 
from the “ first formed blastomere”’ (first micromere a@b?), is erroneous. 
Neither cell can be said to receive a nucleus from the other, for the 
division of the segmentation nucleus, and the formation of the first 
cleavage plane is such as ordinarily takes place in unequal cell division. 
The last statement applies also to all the later cleavages. The micro- 
meres rich in protoplasm, which are later cut off from the yolk-macro- 
mere, cannot be said to give rise to a nucleus which migrates into the 
yolk before complete separation of the “ protoplasmic ” cell. 
