MAMMALIAN SPERMATOGENESIS — OPOSSUM 25 



of double reduction. This is especially apt to be the case where 

 the second spermatocyte cells lie deep within the tubule. On 

 the other hand, when the second spermatocyte cells lie near the 

 periphery of the tubule, there is no trace of such a fusion. The 

 chromosomes are as distinct and separate as one could wish. 



THE CHROMOSOME NUMBER OF OPOSSUM EMBRYOS 



A study of the spermatogenesis has shown that in the male 

 opossum there is a pair of chromosomes which segregate out in 

 the first maturation division and are distributed to the sper- 

 matids in a way comparable to the typical X-Y chromosomes of the 

 insects. From a study of the male alone it could not be defi- 

 nitely stated, for the opossum, which was the X-element, or female- 

 producing component, and which the Y-element, or male-pro- 

 ducing component. In order to clear up this point and to further 

 verify the spermatogonial chromosome counts, a study of the 

 chromosome complex of opossum embryos was undertaken. 



Five very young embryos of unknown sex were used together 

 with three female embryos, whose sex was previously determined 

 by Doctor Hartman. In all cases cold Flemming solution, with 

 urea, was used as the fixative. The dividing cells principally of 

 the nerve cord were used for making counts. 



At first the five embryos of unknown sex were studied. 



Embryo no. 4 showed the condition illustrated in text figure 5, 

 A and B. There are 22 chromosomes in the spindles, two of 

 these are smaller than the rest and are similar to the small ele- 

 ments found in the spermatogonia. Embryo no. 4 clearly pos- 

 sesses the chromosome complex of a male opossum. 



Embryo no. 5 (text fig. 5, C and D) showed, like no. 4, 22 

 chromosomes, including the small X-Y elements. Again, we are 

 dealing with an embryo clearly possessing the male chromosome 

 complex. 



Embryo no. 6 (text fig. 6, K and L) showed consistently 22 

 chromosomes. The small round chromosome was absent, how- 

 ever, and in its place there is a small elongated element similar 

 to the other sex-chromosome. This condition of the chromo- 



