120 bulletin: museum of comparative zoology. 



Chromatin. — Early in the telophase of the last spermatogonium all 

 but one (the accessory chromosome) of the thirty-three small chromo- 

 somes lengthen out into granular filaments and by their union in pairs 

 cause a pseudo-reduction of chromosomes. Upon the reconstruction of 

 the nuclear membrane the chromatin filaments become distributed 

 throughout the nucleus, and it is seen that the pseudo-reduction is 

 accomplished by an end to end union of entire chromosomes during the 

 telophase of the last spermatogonium. 



Meanwhile the accessory chromosome has retained its homogeneotis 

 appearance and is still a simple univalent chromosome. 



With the re-establishing of the nucleus changes begin which result in 

 all the granular chromatin segments becoming massed around the accessory 

 chromosome to form the karyosphere. The karyosphere is not a homo- 

 geneous body of chromatin, but a dense mass of chromatin filaments, in 

 which identity of chromosomes is doubtless preserved. 



In the following prophase the chromosomes arise from the karyosphere 

 by a mere unwinding of the component chromatin filaments. These, as 

 they become free, quickly undergo a longitudinal and later a transverse 

 division, thus forming the cruciform tetrads typical of arthropods. Modi- 

 fications of this form, such as rings and double Y's, are common. 



The accessory chromosome, still homogeneous in appearance, shows 

 evidence of only a single division — the longitudinal one. 



During the first spermatocyte division the sixteen ordinary chromo- 

 somes undergo longitudinal fission, while the accessory chromosome passes 

 into one cell undivided. 



The prophage of the second spermatocyte is of very brief duration, and 

 the following separation of the chromatids of the ordinary chromosomes 

 occurs along the plane of transverse division. But the accessory chromo- 

 some, when present, divides longitudinally. 



Of the four spermatids derived from each primary spermatocyte two 

 possess an accessory chromosome, while the other two do not. 



Centeosome and Arciioplasm. — The archoplasm derived by the 

 breaking down of the spindle of the last spermatogonia] mitosis persists 

 during the growth-period and increases until in the vesicle stage a 

 conspicuous mantle of reticular archoplasm surrounds the nucleus. 



During the entire growth period the centrosome persists as two small 

 dark bodies surrounded by a specialized portion of the archoplasm — the 

 idiosome — resulting from the breaking down of the proximal portion of 

 the astral rays and spindle fibres of the last spermatogonial division. 



Large Spermatocytes. — The first change during the prophase of the 



