28 THEOPHILUS S. PAINTER 



somes was found in all cells of this embryo. The absence of 

 the small rounded chromosome and the presence of a second 

 elongated rod was interpreted as meaning that embryo no. 6 was 

 a female with the 2-X condition. This showed, then, that the 

 small elongated rod was the X-component, and on this basis 

 the rounded chromosome of the male was the Y-component. 



Embryos nos. 7 and 8 (text fig. 5, E, F and G, H) both turned 

 out to be males. 



Thus it will be seen that among the first five embryos studied 

 four were found to possess the chromosome complex of the male 

 while only one showed the female condition. Although the study 

 of the chromosome complex was consistent in that it showed 

 all males to have the X-Y condition and the one female always to 

 have the 2-X condition, a further check was deemed desirable, 

 especially as the material was at hand. 



Doctor Hartman has found that he can identify the sex of 

 the pouch young of the opossum soon after birth, by the presence 

 of the scrotal swellings in the male and the absence of the pouch 

 rudiments and, conversely, in the female the rudiments of the 

 pouch are plainly visible and the rudiments of the scrotal sacs 

 are absent. Doctor Hartman selected for me three embryos 

 which he identified as being of female sex. They were presei^ved 

 in cold Flemming and sectioned. 



Text figure 6, M to R, shows the chromosome complex of these 

 three individuals. The figures show that in every case the 2-X 

 condition of the sex-chromosomes is found just as in embryo no. 6 

 (text fig. 6, K and L) and also that there are 22 chromosomes in 

 all the cells. (Text fig. 6, M and N from embryo no. 10; and 

 P from embryo no. 11, and R from embryo no. 9.) 



In none of the eight embryos did I find any evidence for a 

 fragmentation of the chromosome elements, as Hance did in a 

 similar study of pig embryos. Among the hundreds of ceU 

 plates examined, I invariably found 22 chromosomes. In em- 

 bryos nos, 4, 5, 7, and 8 the X-Y chromosome condition was 

 observed, and in embryos nos 6, 9, 10, and 11 the 2-X condition 

 prevailed. 



