BIGELOW : NUCLEAR CYCLE OF GONIONEMUS MURBACHII. 381 



cyte is much smaller than the primary spermatocyte ; its nucleus during 

 the " resting " stage closely resembles that of the primary spermatocytes ; 

 the achromatic reticulum bears nodal thickenings, and the nucleolus is 

 a purely chromatin structure. Its staining reactions are those typical 

 of other cell generations, and the usual reversal of reaction takes place 

 on the part of the karyosomes in the early prophase. 



Contraction phases are common in the prophase of this as well as of 

 the preceding mitosis, but are believed to be purely artificial. 



The chromatin becomes condensed into an irregular network ; before 

 the breaking down of the nuclear membrane this segments into twelve 

 chromosomes without any intermediate formation of chromomeres. 



The spindles are very small; no " dumb-bells " are formed, since the 

 splitting of the chromosomes is rapid and simultaneous, and the daugh- 

 ter chromosomes are very much smaller than those of the preceding 

 division. 



It is impossible to say certainly whether the division is reducing or 

 equational. 



The centrosome does not disappear in the anaphase, as is the case in 

 all other mitoses in Gonionemus, but persists to form the spermatid 

 centrosome. 



Metamorphosis of the spermatids (Figs. 88-108). — The spermatid 

 centrosome, which lies at the cell margin, divides, and one-half migrates 

 inward to the nucleus, finally being flattened against the nuclear mem- 

 brane. It remains connected with the distal centrosome by an axial 

 filament. The tail filament is believed to be an outgrowth of the distal 

 centrosome. 



The interzonal bridge connecting pairs of daughter spermatids breaks 

 down. The centrosome migrates along the margin of the cell until it 

 comes to lie in the prolongation of the long axis of the nucleus. The 

 remnants of the interzonal filaments remain attached to the nucleus, but 

 the remnants of the polar portion of the spindle always lie in the neigh- 

 borhood of the centrosome. 



The interzonal remnants disappear, and afterwards two archoplasmic 

 masses are to be seen in place of the polar remnants, one lying on either 

 side of the axial filament. These increase in size, and form the sheath 

 of the middle piece. 



The acrosome is a sphere of archoplasm, which arises after the disap- 

 pearance of the interzonal remnants ; its origin is doubtful. 



In addition to the normal spermatid there are spermatids showing 

 abnormalities of two kinds caused by incomplete spermatocyte divisions. 



