MAMMALIAN SPERMATOGENESIS OPOSSUM 17 



was forced to bubble through the solutions containing the 

 material. After using Flemming, Hance's suggestion of clearing 

 with cedar oil directly from 95 per cent alcohol was followed. 



The embryos employed were both unborn and pouch young. 

 Only the cold Flemming method was used for preserving this 

 material. 



Embedding was carried on in the usual way and sections cut 

 at from 6 to 8^. Iron-haematoxylin was the method used for 

 staining. 



The author is indebted to Dr. Carl Hartman for much of the 

 living material used for the present work, and also for the priv- 

 ilege of examining some of his preparations of polar spindles, 

 which will be found figured on page 36. 



SPERMATOGONIAL DIVISIONS 



Dividing spermatogonial cells, in equatorial plate view, show 

 22 chromosomes (figs. 1 to 6, see also somatic divisions, fig. 5, on 

 page 26). Of these 21 are elongated rods, of various sizes, bent 

 or shaped in characteristic ways, and one small rounded chro- 

 mosome. There are no typical V-shaped chromosomes in the 

 opossum complex. Usually the twenty largest chromosomes 

 form a ring about the two smallest (figs. 1 to 4, also somatic 

 divisions), but occasionally this condition does not obtain (figs. 

 5 to 6). On inspection of the figures it will be noted that there 

 are a number of pairs of chromosomes, similar in size and shape; 

 and, further, that the two chromosomes lying in the center of the 

 spindle are decidedly smaller than the remaining twenty elements. 

 This fact is important and is shown in all figures (figs. 1, 2, 3, 4, 

 and 6) where there is no great foreshortening of the chromosomes. 

 It is brought out, best, perhaps, by arranging the chromosomes of 

 any spindle in their approximate size relations. In text figure 1 

 the chromosomes of the cells shown in figures 2, 4, and 6 are 

 copied by the aid of a copy camera lucida, and arranged in 

 approximately the order of their size. It will be seen, first, that 

 the twenty larger chromosomes can all be paired up, with more 

 or less accuracy, and, secondly, that the two smallest chromo- 

 somes not only are decidedly smaller, but that they have no 



