330 bulletin: museum of comparative zoology. 



karyoplasra. The nucleolar shell contributes to the chromatin struc- 

 tures, but the central plasmosouie persists uutil the breaking down of 

 the nuclear membrane. The chromatin becomes condensed into forty- 

 eight spherical chromomeres, which are connected by linin strands. 



After the breaking down of the nuclear membrane the twenty-four 

 chromosomes are formed by the union in pairs of the chromomeres. 

 Their constricted form results from this double origin. In division each 

 chromomere is divided lengthwise. 



The achromatic figure, which is formed during the metaphase, has no 

 astral radiations. Though the centrosome, which is a minute granule, 

 disappears in the late anaphase, the stout interzonal filaments often 

 hold the daughter cells for a long time in connection. 



The chromosomes do not pair in the telophase, but become connected 

 into an irregular network, and eventually break up into their component 

 granules. 



The nucleolus is formed by a condensation of the chromatin granules. 



Primary spermatocytes (Figs. 39-70). — In the "resting" stage the 

 chromatin is diffused. The nucleolus is not compound, but is a purely 

 chromatin structure, which breaks up in the prophase and contributes 

 to the formation of the chromatin net. The staining reactions of the 

 nucleus are those typical of somatic cells and spermatogonia, except 

 that the entire nucleolus combines with basic dyes. During the pro- 

 phase contraction of the chromatin structures often occurs, but this is 

 believed to be artificial and not to represent a normal stage in sperma- 

 togenesis. 



In the normal prophase the chromatin becomes condensed into a 

 smooth, homogeneous net of few strands, which, instead of being meta- 

 morphosed into a continuous spireme thread, probably segments into 

 twenty-four chromatin masses. Upon the disappearance of the nuclear 

 membrane these masses pair, forming twelve chromosomes. 



Apparently there is no formation of tetrads. 



The spindles of the first maturation mitosis are of two types. In the 

 more usual one the chromosomes become dumb-bell-shaped and divide by 

 a gradual drawing apart of their thickened ends. In the rarer type 

 division is rapid and irregular, and "dumb-bells" are not formed; but 

 in both the daughter chromosomes are much larger than those of the 

 spermatogonial division. 



It is impossible to say certainly whether the first maturation mitosis 

 is an equation or a reduction division. 



Secondary spermatocytes (Figs. 71-87). — The secondary spermato- 



