SPERMATOGENESIS IN THE ALBINO RAT 141 



parable with animals in which the chromosomes have the same 

 final shapes, the histories of which have been worked out (e.g., 

 the Orthoptera; McClung, '14, and Wenrich, '16), so that one 

 may say that the history of the rat chromosomes during this 

 period corresponds with that of the similarly shaped and con- 

 stituted chromosomes of those forms which are so much more 

 favorable for study. This topic is further treated under the dis- 

 cussion, page 159. 



Diakinesis. The beginning of differentiation into the shapes 

 assumed by the chromosomes is seen after the diplotene stage has 

 passed and diakinesis is occurring. Threads begin to separate 

 longitudinally, when either th^ ends remain attached (fig. 34) 

 or spread apart (figs. 30 and 31). In the latter case adherence 

 is maintained near the ends. Single, double, and even more 

 complex rings (figs. 27, 30, and 34) are thus formed. The 

 threads are delicate in structure, with the chromomeres quite 

 well separated, as shown in the photographs, figures 47 and 48. 

 At the places where the threads in the double rings apparently 

 cross, their duplex constitution may sometimes be made out, 

 as illustrated in figure 34. The tetrad condition of the chromo- 

 some is thus evident in the early prophase. Later stages con- 

 firm this evidence. The exact time when the longitudinal 

 splitting of the homologous chromosomes takes place has not 

 been determined. It seems to be accomplished by the time that 

 the shapes are established. 



Further treatment of the changes occurring in the later period 

 follows under the topics: 'The shapes of the first spermatocyte 

 chromosomes,' 'The composition of the first spermatocycte 

 euchromosomes,' 'The number and size of the first spermatocyte 

 chromosomes,' and 'The accessory.' 



The shapes of the first spermatocyte chromosomes. The only 

 shapes assumed are the rod, the cross, and the various types of 

 the ring — single, double, and triple. These are shown in figures 

 18, 19, 27, 28, 29, 30, 31, 50 to 52, 54 to 57. Some interchange 

 seems to occur in the number of rings and crosses, indicating 

 that the individual chromosomes do not always assume the same 

 final shapes. Thus in some cells in prophase two crosses of nearly 



