92 



Mr. L. Doncaster. 



[Sept. 21, 



resting condition, but in others chromatin is beginning to appear, and where- 

 it is aggregated into distinct masses the number of these approaches 20, but 

 they are too irregular for an accurate count to be possible. 



The testes in the young pupae are divided into follicles, in the walls of 

 which are very large nuclei with many chromatin granules. In each follicle 

 the cells are nearly, but not quite, exactly at the same stage of development. 

 This fact makes it very difficult to determine with certainty the exact course 

 of events, for only here and there does one find a follicle in which slightly 

 different stages are represented, and so get a clue as to the course of 

 development. In pupse which are not too far advanced, various stagefj 

 occur in different follicles of the same testis from primary spermatocytes 

 before the maturation divisions have begun, up to spermatids which are 

 beginning their metamorphosis into spermatozoa. By comparing a number 

 of testes of this age with one another, I have found a continuous series 

 of stages, and think there is little doubt that in the account which follows 

 they are placed in the correct order. The difficulty is increased by the fact 

 that stages which appear closely similar occur at considerable intervals, and 

 great care is needed in distinguishing one from the other. 



The primary spermatocytes are rounded cells lying free in the cavity 

 of tlie follicle ; each has a large vesicular nucleus, containing a very faintly- 

 staining reticulum, and often a nucleolus (Plate 1, fig. 5 a, h). At the 

 approach of the " meiotic " divisions chromatin appears as small dots close- 

 to the nuclear membrane ; these dots coalesce into irregular masses just 

 inside the membrane, and these then form themselves into 10 chromosomes, 

 which in some cases for a short time have an elongated band-like form 

 (Plate 1, fig. 6). The cell is meanwliile becoming elongated and pear-sliaped, 

 the nucleus occupying the wider end. At the apex of tlie narjr,pw~end a 

 minute black dot, the centrosome (centriole), may often be sefen, and at 

 a slightly later stage a second centrosome is seen at the broad end of the 

 cell, close to the nucleus (fig. 7 f<). I have not observed their origin. In 

 the primary spermatocyte, when tlie chromatin is beginning to appear, a 

 slightly larger black dot appears outside the nucleus, but as this may still; 

 Vje seen, in addition to the centrosoines, at the stage described, it cannot give 

 origin to the centrosomes. 



The chromosomes now become shorter, and place themselves in a radial 

 arrangement, as in an equatorial plate; the nuclear membrane persists, but 

 the nucleus is drawn out towards the narrow apex of thv. coll, so that it also 

 becomes somewhat pear-shaped (Plato 1, fig. 7 <■). At tliis stage, in some 

 cells the narrow apex of the cell is elongated into a fine process, witli the 

 centrosome (centriole) at its ti]) (fig. 7 like that figured by Mark and 



