248 FIELD. [Vol. XI. 



nucleus remains homosreneous under the same reasjents and 

 conditions which demonstrate the chromosomes in the others. 

 These spermatozoa with the homogeneous nuclei are the most 

 active and most frequently penetrate into the ovum. In sec- 

 tions of fertilized eggs the nucleus of the spermatozoon when 

 in the outer zone of the cytoplasm of the ovum is small, dense, 

 and homogeneous (Fig. 56) ; on the other hand the one which 

 has traveled some distance toward the female pronucleus is 

 considerably larger, and shows the eight or nine chromosomes 

 surrounded by a lightly staining caryoplasma (Figs. 57, 58). 

 Hence it is probable either that the chromosomes in the 

 nucleus of the spermatozoon dissolve in the caryoplasma and 

 form a denser homogeneous mass preparatory to penetrating 

 the membranes and more compacted outer cytoplasmic layer of 

 the ovum, or else the caryoplasma is extruded and the nucleus 

 of the mature spermatozoon consists very largely or even solely 

 of chromosomes (nuclein) closely packed together. There i^ a 

 very considerable reduction in the size of the nucleus in chang- 

 ing from that of the spermatid to that of the spermatozoon, and 

 correspondingly an increase in size, with a reappearance and 

 wider separation of the same number of chromosomes, after the 

 spermatozoon in the fertilization process has passed the 

 peripheral denser portion of the cytoplasm of the ovum. The 

 question is whether this change in the nucleus is merely one 

 of density, i.e., (i) does the nucleus of the mature spermatozoon 

 consist of the same quantity of nuclein and caryoplasma, but 

 with the nuclein dissolved in the caryoplasma, or (2) does the 

 caryoplasma as a liquid portion pass into the cytoplasm before 

 the spermatozoon becomes fully mature, and after the sperma- 

 tozoon has entered the cytoplasm of the &g% is this caryoplasma 

 restored from the cytoplasm of the egg .-• On account of the 

 very evident alterations in size of the nucleus, one is inclined 

 towards the latter alternative. It seems to be a pretty good 

 case for proving that an interchange of substance goes on 

 between the nucleus and the cytoplasm, and that in this in- 

 stance at least the substance which passes through the nuclear 

 membrane is the caryoplasma, a liquid protoplasm. If this is 

 the case, it makes strongly for the view that the nuclein is the 



