MAMMALIAN SPERMATOGENESIS — OPOSSUM 37 



SUMMARY 



There are 22 chromosomes in the spermatogonia of the opos- 

 sum. The two smallest of these constitute a typical X-Y sex- 

 chromosome complex, which can be followed through the growth 

 period of the first spermatocyte. In the first maturation spindle 

 there are 11 chromosomes — 10 tetrads and the X-Y chromosome. 

 The X and Y components segregate from each other in this 

 division so that the secondary spermatocytes contain either 10 

 autosomes plus X, or 10 autosomes plus Y. There are 11 chro- 

 somes in all secondary spermatocytes, of these one is either the 

 X-component or the Y-component. In either case the sex chro- 

 mosome divides equationally. Half the sperm carry the X- and 

 half carry the Y-chromosome. 



A study of dividing somatic cells of embryos showed that in 

 both male and female embryos 22 chromosomes were present. 

 In the males the X-Y condition was found, while in the females 

 the 2-X condition existed. 



It is concluded that Jordan's failure to find more than 17 chro- 

 mosomes in the spermatogonia was due to faulty fixation. The 

 'accessory chromosome' which he described was not the true 

 sex-chromosome, but probably a displaced tetrad. Hartman's 

 error in count was due to the precocious spUtting of the X-chro- 

 mosome tetrad in the polar spindles. 



BIBLIOGRAPHY 



Allen, Ezra 1916 Experiments in technique. Anat. Rec, vol. 10. 



1918 Spermatogenesis in the albino rat. Jour. Morph., vol. 31. 



1919 A technique which preserves the normal cytological conditions 

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Bachhuber, L. J. 1916 The spermatogenesis of the rabbit. Biol. Bull., vol. 30. 

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1910 Accessory chromosomes in man. Biol. Bull., vol. 19. 



1916 Studies on the chromosomes of the common fowl as seen in 

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Hance, Robert T. 1917 The fixation of mammalian chromosomes. Anat. 

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1917 The diploid chromosome complexes of the pig. Jour. Morph., 

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