72 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



With regard to the male pronucleus Groom ('94, p. 134) states : 

 " Sections made of ova of Lepas auatifera 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 prouucleus 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 spermatozoon 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, new 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 



