SPERMATOGENESIS OF THE PIG. 9 



Wilson ('09) found in Syromastes that half of the spermatids 

 possess two more chromosomes than the remainder. It was 

 predicted by him that in consequence the somatic cells of the 

 female of this species would show two more chromosomes than 

 the somatic cells of the male, and later the facts were found to be 

 in exact accord with his predictions, the somatic cells of the female 

 of Syromastes having been found to contain twenty-four, those of 

 the male, twenty-two chromosomes. A similar condition has 

 been found in other tracheates, but dimorphism in the number of 

 chromosomes in the germinal and somatic cells of the two sexes 

 among the vertebrates has thus far been only inferred, not 

 actually demonstrated. 



The present study on the spermatogenesis of the pig was taken 

 up at the suggestion of Professor M. F. Guyer, to whom I am 

 much indebted for many helpful suggestions during the progress 

 of the \vork. Points of special interest in this paper are as 

 follows : 



1. The presence of a distinct pair of accessory chromosomes. 



2. The resulting dimorphic condition in the spermatozoa of 

 the pig. 



3. Dimorphism in the number of chromosomes in both the 

 germinal and somatic cells of the male and female animals. 



4. The abundance of large conspicuous interstitial cells in the 

 testes. 



5. The second reduction of the chromosome number in the 

 secondary spermatocyte, which was found to be simply equa- 

 tional. 



6. The throwing off of a large mass of cytoplasm, containing 

 one of the centrosomes, at the time of the final development of 

 the spermatozoan . 



MATERIAL AND METHODS. 



I have been very fortunate in obtaining from several sources 

 exceptionally good material for this investigation. The major 

 part of the material studied was obtained from a vigorous Poland 

 China boar about ten months old. The animal came from 

 registered stock and was the property of the College of Agri- 

 culture of the University of Wisconsin. The material was ob- 



