BLACKMAN: THE SPERMATOGENESIS OF SCOLOl'li.M'KA. 



31 



considered, this behavior is naturally expected. Owing to the compact 

 manner in which the extremely line chromatin threads are aggregated 

 in tiie karyosphere, only a few chromosomes can arise from it at one 

 time, so that naturally those which are first formed are in more ad- 

 vanced stages of development than those that appear later (Fig. 23). 

 However, by the time the centrosomes are upon opposite sides of the 

 nucleus and the nuclear membrane has begun to disintegrate, all of the 

 chromosomes are in apparently the same stages (Figs. 29, 30). All are dense, 

 homogeneous structures, 



which take the stain with . ■■' V V' \- ' : .. .;•" 



avidity and retain it strongly. 

 While the other chromo- 

 somes have been undergoing 

 these changes in structure, ";• 

 the accessory chromosome 

 has also changed in shape. 

 It has apparently at no time 

 lost its homogeneous struc- 

 ture, but that it has under- 

 gone important changes is 

 evident. It is no longer a 

 spherical body, but now has 

 the form of a rod, the ends 

 of which are slightly notched 

 (Figs. 29, 30, Fig. C). These Fig. C. Nucleus of first spermatocyte 



notches I believe indicate a during late prophase, showing various modi- 

 longitudinal division. The neations in the shape of the chromosomes 



, ,. a . at this time. The accessory chromosome is 



accessory chromosome diners , . , .. , n . o mna 



J seen to be notched at the ends. Centrosomes 



from the other chromosomes with we ll-developed astral rays at opposite 

 in shape and is evidently not poles of the nucleus. X 1,440 dia. 

 of a tetrad nature. "When its 



origin is considered this difference in form is very easily explained, and 

 is indeed exactly what would be expected. The other chromosomes are 

 formed by a fusion of two of the spermatogonial chromosomes into one 

 body during the telophase of the last mitosis of the division period. On 

 the other hand the modified accessory chromosome is obtained directly 

 from a single chromosome of the spermatogonium. This being true, it 

 woidd be but logical to expect it to act differently. The primary object in 

 the division of the spermatocyte is the reduction of the chromosomes to 

 one half the somatic number. This is generally if not invariably accom- 



Fig. C. 



