THE SPERMATOGENESIS OF DOMESTIC MAMMALS. 359 
MoNnTGoMERY (11b) succeeded in tracing directly the formation of 
Sertoli-cells from the antepenultimate spermatogonia in man. The close 
resemblance of Sertoli-cells and spermatogonia, and their relative number, led 
BacHHUBER (’16) to the conclusion that the nutritive cells in the rabbit arise 
from the primordial germ cells. 
During the early prophase of the spermatogonia numerous chromatin 
masses appear which gradually increase in size. The nuclei in this stage 
greatly resemble those of the interstitial cells, both in appearance and in size 
(Fig. 3). Fig. 9 represents a prophase of a primary spermatogonium. The 
chromosomes here are variously curved and somewhat elongated but one of 
them usually appears round or oval in form. 
In the equatorial plates of the metaphase of the spermatogonia the 
chromosomes have a tendency to collect together and form a mass, so closely 
overlying one another that it is not possible to identify the individual 
chromosome (Figs. 4, 10). From the preparations of these stages, it is 
impossible to obtain a definite conclusion with regard to the number and the 
condition of the chromosomes. In favorable preparations I was able to count 
over thirtythree chromosomes, but never more than thirtyeight. Thus, 
although it is difficult to determine the accurate number of chromosomes, it 
is easy to distinguish many symmetrical pairs as shown in Fig. 4. In the 
metaphase all the chromosomes arrange themselves at the equatorial plate, 
where they simultaneously divide into two portions; no special chromosome 
with different behavior is to be seen among them (Figs. 4, 11,12). During 
the prophase, as mentioned above, one of the chromosomes usually appears 
round or oval in form and this may represent the future accessory chromo- 
some (Fig. 9). It is, however, impossible to establish direct continuity between 
the chromatin nucleolus of the spermatogonium and the future accessory 
chromosome. 
As to the essential achromatic structure of the spermatogonia, satisfactory 
preparations are very difficult to obtain. In the earlier stage of the sper- 
matogonia, the cell boundaries are indistinct and the amount of the cytoplasm, 
especially in the secondary spermatogonia, is very small. The idiozome rarely 
appears, and the centrosome as well as the other cytoplasmic structures can 
not be identified (Figs. 7, 8). 
