72 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
With regard to the male pronucleus Groom (94, p. 134) states: 
‘Sections made of ova of Lepas anatifera before or shortly after the 
formation of the first polar body show the first directive spindle or a 
small round nucleus with several chromatin elements.” Having failed 
to find the male pronucleus, he concluded that it ‘“‘must be exceedingly 
small and easily overlooked, otherwise it would be necessary to conclude 
that the fusion of the two pronuclei takes place immediately after the 
first polar body is formed (in which case it would be very rarely detected 
in ova which had given off the first polar body); but this seems improb- 
able, though traces of a male pronucleus were never found in sections at 
any later phase even in ova where the second polar body was being or 
had just been given off.” 
Some of these observations by Groom are in accord with my statement 
that the male pronucleus has not been certainly identified in sections 
corresponding to a stage earlier than that represented in my Figure 3, 
although the spermatozo6n is probably present at a stage earlier than 
that represented in Figure 1, in which the second polar cell has just 
been separated. Groom’s supposition that the pronuclei fuse soon after 
the formation of the first polar cell is opposed by the evidence afforded 
by my Figures 17-21. It will be shown later that Groom probably saw 
the male pronucleus in these later stages, but misinterpreted it as one 
of the daughter nuclei resulting from the first division of the egg. 
Groom says (p. 135), ‘‘ The nucleus, which, during the period at which 
the ovum was undergoing contraction [yolk-lobe stages], was small and 
situated peripherally and anteriorly [at animal pole], and was invisible 
without special preparation, ncw becomes larger, and appears as a defi- 
nite clear spot.” He further states (p. 137) that, ‘‘the clear spot 
appearing with the separation of the protoplasm is almost certainly the 
segmentation-nucleus.” I have seen this “‘ clear spot,” and sections show 
that it is the female pronucleus, or sometimes the two pronuclei so ap- 
proximated that viewed through the opaque substance of the living egg 
the appearance is that of one transparent area. Groom’s statements 
regarding these stages were apparently based upon studies of living eggs, 
which are so opaque as to render observation difficult and uncertain. 
In a stage which Groom interpreted as that of the first cleavage, he 
found * two nuclei in the newly-formed [first] blastomere ” ; these were 
regarded as the daughter nuclei of the first segmentation nucleus 
(pp. 137, 142, 145). In the review of literature on first cleavage it will 
be pointed out that Groom apparently has mistaken for the first segmen- 
tation of the ovum a maturation phase, such as that represented in my 
