MAMMALIAN SPERMATOGENESIS — OPOSSUM 21 



often sees clearly the deeply staining chromatin masses labeled 

 X and Y. Following this period, the chromatin threads expand 

 until they fill the entire nucleus and the chromosomes become 

 woolly in appearance (text fig. 2, D and E). The whole cell 

 remains in this phase for a considerable time. In early stages 

 (text fig. 2, E) three nucleoli are found, but the homologies of 

 these with those described in the early growth period (text fig. 

 2, A) are not clear. As this stage advances, however, two of the 

 nucleoli seem to disintegrate, they swell, lose their affinity for 

 the stain, and eventually disappear from view. Some indication 

 of this may be seen in text figure 2, E. The third nucleolus 

 persists and frequently has a form (text fig. 2, D) which suggests 

 that it is made up of the X and Y chromosomes lying side by 

 side. The contraction of the diplotene threads follows the dis- 

 appearance of the two nucleoli, and the tetrads of the first matura- 

 tion spindle take on their characteristic forms. 



In the first maturation division spindles, there are 1 1 chromo- 

 somes (figs. 7 to 11). It has proved difficult and unsatisfactory 

 to determine the number of chromosomes present from ordinary 

 equatorial plate views. The reason for this is not because of a 

 fusion of the elements, but because of the shapes of the chromo- 

 somes themselves, as a glance at the figures will show. In side 

 views of the spindles, particularly just as the tetrads are entering 

 the spindles, and in slides in which destaining has been carried 

 out to an advanced degree, one can usually make out ten tjrpical 

 tetrads and an eleventh chromosome which is tripartite in form, 

 that is, made up of two egg-shaped elements, and a blunt rod 

 joined end to end. This is the X-Y chromosomes complex. (In 

 fig. 10 two of the chromosomes, which lie underneath the rest 

 are drawn out at one side.) 



The ten tetrads have such characteristic forms, that after 

 experience one can usually identify all of them and determine 

 which are lacking, in case the spindle has been cut in two. In 

 text figure 3 the typical shapes of these tetrads, as taken from 

 two cells (figs. 7 and 10), are shown. A comparison of the 

 chromosomes figured in text figure 3, with the spindles in figures 

 7 to 12, will enable the reader to identify most of the elements 



