THE SPERMATOGENESIS OF DOMESTIC MAMMALS. 397 
VI. Summary. 
1. In the testes of embryos and quite young animals the spermatogonia 
are divided by amitosis, and in such young individuals amitosis occurs more 
frequently than mitosis. Judging from their nuclear organizations and other 
structures, it is evident that the cells produced by amitosis are degenerating, 
being used as nutritive materials by the germ cells. 
2. The resting nuclei in both the spermatogonial generations, the 
penultimate and the ultimate spermatogonia, usually contain one large 
nucleolus and a small chromatin mass. 
3. In the spermatogonia the number of chromosomes may be counted 
as thirty three. These vary considerably in size and form, but are found to 
be in pairs. Each chromosome simultaneously divides into two portions along 
the longitudinal split which first appeared in the spireme stage; no special 
chromosome with different behavior is to be seen among them. 
4. In the telophase of the ultimate spermatogonia the chromosomes are 
not fused, and thus it is possible to make out individual chromosomes in 
which the longitudinal split is rarely found: Subsequently the chromosomes 
become lengthened to the leptotene threads. 
5. Conjugation of chromosomes probably takes place by telosynapsis. 
during the synaptene stage. In this stage the leptotene threads converge 
towards one side of the nucleus, leaving a clear space on the other side. 
6. During the final prophase the longitudinal split and transverse con- 
striction of the chromosomes are found. 
7. The chromosomes are divided along the constricted point in the first 
reduction division which is the reducing division. 
8. In the second reduction division all the chfomosomes become so 
placed that the line of the longitudinal split coincides with the equatorial 
plane, and along this line all the chromosomes as well as the accessory ones 
are divided at the same time, thus it is simply an equation-division. 
9. In the spermatogonia and the spermatocyte the centrosome is so 
minute that it can not be distinguished from the other granules. The 
changes of the centrosome during the formation of the spermatozoa are 
considerably different from those observed in the horse. -The centrosome 
