312 J. E. WODSEDALEK. 



4. In the secondary spermatocyte division the heart-shaped 

 accessory passes undivided to one pole in advance of the other 

 chromosomes. 



5. The primary spermatocyte division is evidently the reduc- 

 tion division, giving rise to two different types of secondary 

 spermatocytes; one with the accessory and the other lacking it. 



6. There is no resting stage following the first spermatocyte 

 division. 



7. A second pairing of the chromosomes takes place so that 

 only one-fourth the original number of chromosomes appear for 

 division in the secondary spermatocyte. 



8. The accessory chromosome divides in the secondary sper- 

 matocyte division a little in advance of the other chromosomes 

 the same as it does in the spermatogonia. 



9. The one type of secondary spermatocyte, which contains 

 the accessory, gives rise to two spermatids, each containing the 

 accessory and nine bivalent chromosomes. 



10. The other type of secondary spermatocyte, which lacks 

 the accessory, gives rise to two spermatids, each containing only 

 the nine bivalent chromosomes. 



11. In terms of univalence, then, one type of spermatid re- 

 ceives eighteen chromosomes plus the accessory and the other 

 type receives only the eighteen ordinary chromosomes. 



12. In view of the foregoing facts, two different types of 

 spermatozoa, equal in numbers, are produced in the horse; the 

 one type contains in addition to the ordinary chromosomes the 

 accessory, and is apparently the female determining spermato- 

 zoon. 



13. Actual measurements of six hundred mature spermatozoa 

 reveal the interesting fact that two distinct types of spermatozoa 

 as regards size are produced, the one being much larger and 

 presumably the one which bears the accessory chromosome. 



14. The dimorphic condition among the spermatozoa of the 

 horse lends additional support to the chromosome theory of sex 

 determination. 



15. The developing spermatozoa invariably cast off a mass of 

 cytoplasm. 



16. A chromatoid body, which simulates the appearance of a 



