THE SPERMATOGENESIS OF UMBKA I.IMI. 125 



more active multiplication period, do not show such conspicuous 

 gradations, although they do show the chromosomes to be in 

 the shape of V's and L's. The writer believes that the condition 

 as shown in Figs. 12 and 13 is unusual. Such figures are found 

 in small numbers and usually occur at the periphery of a lobule 

 or cyst and in small isolated clusters in the ends of the testis. 

 The origin of the chromosomes in the form of V's and L's seems 

 to be the more usual occurrence. The more unusual type of 

 chromosomes have been of service in this study, however, since 

 they furnish valuable material for checking chromosome counts 

 made in other cells. Where the chromosomes appear as they 

 do in Figs. 5 to n, counting is difficult because they are not all 

 in one plane. Some lie above the others, often in as many as 

 three planes, and such an arrangement makes an accurate count 

 often very difficult. 



The chromosomes in the large spermatogonial cells are so 

 clear that it is not difficult to pick out the homologous pairs. 

 With this purpose in mind the chromosomes of Figs. 6 to 10, 

 have been listed in Figs. 56 to 60 according to size. It will 

 be noticed in all of these figures that although the chromosomes 

 vary somewhat in shape at different times, two of them, labeled 

 (a) in Figs. 6 to 10 and Figs. 56 to 60, although differing in size 

 in the different cells have a fairly constant and characteristic 

 shape. The members of this pair may be found side by side but 

 more commonly one or two other chromosomes intervene. The 

 fate of the chromatin nucleolus has not been fully determined 

 in this study. Other investigators have shown that certain so- 

 called nucleoli really constitute the sex chromosomes. Painter 

 ('24), says: "It is concluded that the chromatin nucleolus is 

 made up of the sex chromosome element in the opossum and pre- 

 sumably in all other mammals." The nuclei of the spermatogonial 

 growth period and those at the end of the multiplication period 

 (Figs. 15 to 17) in Umbra may show chromatin nucleoli vary- 

 ing from one to three. One is of more constant occurrence (Fig. 

 15), however, and it is in only a minor number of instances that 

 two nucleoli are found in those spermatogonial cells which go 

 into the maturation divisions. The relative constancy in size and 

 shape of the two chromosomes just referred to, in all the spermato- 



