SPERMATOGENESIS OF THE PIG. 29 



4. Two large nucleoli are present in the resting stages of the 

 spermatogonia. These can be traced through the entire sper- 

 matogenesis of the pig and are apparently correlated with, or are 

 the same thing as the two accessories. Smaller nucleoli, usually 

 two, are also present. 



5. Eighteen rod-shaped chromosomes differing somewhat in 

 size occur in all of the spermatogonia where a definite count could 

 be made. Two of these, undoubtedly the accessories, can usually 

 be seen to one side of the main mass of chromosomes. 



6. During the spermatogonial division the accessories divide 

 and occasionally pass to the poles in advance of the other chro- 

 mosomes. 



7. The last spermatogonial division gives rise to cells which 

 through a process of growth become the primary spermatocytes. 

 Both nucleus and cytoplasm increase greatly in size. Synizesis 

 followed by synapsis occurs. 



8. An apparently continuous spireme is formed which later 

 breaks up into U-shaped and variously curved chromosomes. A 

 centrosphere containing the centrosome is present. 



9. The two large round nucleoli which remain very conspicuous 

 throughout the process of growth of the primary spermatocyte 

 come together and elongate during the late prophase. 



10. Ten chromosomes appear for division in the primary 

 spermatocyte of which eight are evidently bivalent and two 

 accessory. The accessories are usually considerably to one side 

 of the other chromosome. 



1 1 . The two accessory chromosomes which are out of the main 

 spindle pass undivided to one pole in advance of the other chromo- 

 somes. 



12. The primary spermatocyte division is evidently the reduc- 

 tion division. It gives rise to two cells, one of which contains 

 ten (eight autosomes plus the two accessories) and the other only 

 eight chromosomes. 



13. The chromosomes in these daughter cells are larger than 

 the univalents of the spermatogonia and show some indications 

 of bivalence. 



14. The secondary spermatocytes formed by the last division 

 ordinarily go into a resting stage. In half of these the two 



