1904.] NATURAL SCIENCES OF PHILADELPHIA. 743 



The first pole-body is cut off by the egg shortly after the expulsion 

 from the body, and when it is cut off the two cell-membranes separate 

 it from the surface of the egg (fig. 9). Shortly afterward these two 

 jnembranes swell, probably by osmotic action of the surrounding water, 

 and together compose a double membrane removed from the surface 

 of the egg (fig. 10), while the cytoplasm has formed a third delicate 

 membrane which remains adherent to it. By this means the first polar 

 body becomes removed from the surface of the egg. The ovocyte of 

 the second order (fig. 10) shows the second polar spindle, like the first 

 in form, and without an intervening rest stage, and this also contains 

 seven chromosomes (fig. 11). The chromosomes are too minute to 

 allow a determination of which of these is the reduction division ; the 

 second polar body is then cut off (fig. 12), but remains adherent to the 

 egg, and the stage of the ovotid is reached. 



During the process of formation of the second polar body the sper- 

 matozoon is changing into the sperm nucleus (figs. 10, 12), in that its 

 head becomes a rounded chromatin mass lying within a clear vacuole. 

 Quite frequently there is polyspermy, but I have no evidence that such 

 cases develop into embryos. When the second polar body is cut off 

 we find the sperm nucleus in the form of a large spherical nucleus, with 

 a nucleolus and a linin network ; and the egg nucleus in a less advanced 

 stage (fig. 13). Finally the two nuclei are seen, both in the rest stage 

 (fig. 14). It is remarkable, and to my knowledge unique in the known 

 cases of fertilization, that the two pronuclei are unequal in volume 

 in the rest stage (fig. 14), as also in the prophases of the first cleavage 

 (fig. 15); not a single case was found in which they were of equal 

 volume. The smaller one appears to be the egg nucleus, because in 

 most cases it is the one nearest the second polar body. Its smaller 

 size is probably due to the fact that it is formed in less time than the 

 sperm nucleus, and that the period is very short between its rest stage 

 and the formation of the first cleavage spindle. 



Fig. 15 shows the two pronuclei in the prophase of the first cleavage, 

 the sperm nucleus being the one at the left hand ; the nuclear membrane 

 of each has disappeared at the point where there is an accumulation 

 of finely structured cytoplasm, resembling the "archoplasm" of the 

 similar stage in Ascaris. Each pronucleus contains a linin network 

 and seven minute chromosomes ; these are shown in the drawing much 

 less distinctly than in the preparation, where the chromosomes are 

 stained intensely blue and the linin very faintly. The first cleavage 

 spindle (fig. 16) contains 14 chromosomes (only 12 seen in this figiu-e), 

 and the spindle, unlike the polar spindles, is pointed at each end ; but 



