188 W. E. AGAR 



a compact rounded mass in sharp contradistinction to the still 

 thread-Hke autosomes (PI. 12, figs. 8-10). From its first appear- 

 ance onwards it is attached to the end of one of tlie autosomes. 

 At first it is not possible to identify the minute Y, but later, as 

 the autosomes lose in staining capacity, Y becomes conspicuous 

 by reason of its denser stain. At first it is distinct from X, 

 but they soon fuse to form a bivalent (PI. 12, fig. 9). 



A large pale plasmosome makes its ajDpearance at the time 

 that the sex chromosomes are uniting and in close contact with 

 them. The nature of this plasmosome, and its relation to the 

 plasmosome of the earlier stages, is discussed below. 



In the late pachytene stage (PI. 12, figs. 9-10) the staining 

 capacity of the autosomes becomes greatly diminished, and 

 their outlines become somewhat blurred by the development 

 of outgrowi:hs and anastomoses between the different chromo- 

 somes. The compact XY bivalent is now very conspicuous, 

 owing to the fact that the general decrease in staining capacity 

 does not affect it nearly so much as the autosomes. 



Fig. 11 represents the diplotene stage. This is perhaps 

 chiefly interesting on account of the confidence wdth which 

 its mode of derivation from the pachytene stage can be deter- 

 mined. In many late pachytene nuclei, such as about the stage 

 shown in fig. 10, the five pachytene bands can be counted 

 wdth ease and certainty, and one can follow' step by step in 

 great detail the conversion of each of these bands into one of 

 the diplotene bivalent s by the appearance and gradual widening 

 of a longitudinal split down its middle. The three stages 

 figured (PI. 12, figs. 10, 11, 12) will, however, probably be 

 enough to carry conviction that the gemini of the diplotene 

 nucleus are derived from the pachytene bands in this way, and 

 not by their doubling over as is required by the theory of 

 telosyndesis. 



Fig. 13 shows a stage in the contraction of the diplotene 

 loops into the definitive bivalents. The nuclear membrane 

 has by now disappeared. The great increase in bulk of the 

 chromosomes which has taken place between the stages shown 

 in figs. 11 or 12 and that shown in fig. 13 is remarkable. A 



