374 Vniversity of California Publications in Zoology [Vol.16 



It would seem from the examination of the testes of mice of differ- 

 ent ages that the whole process of spermatogenesis might be worked 

 out in the mature testis and on a few slides, since all stages from the 

 spermatogonia to the mature spermatozoa are found in sections of the 

 tubules of the adult mouse. Such a condition offers a great advantage 

 over the study of mammalian eggs, because in this work a great 

 number of specimens is needed and in many cases specimens of differ- 

 ent ages. 



THE PRIMARY SPERMATOCYTES 

 The spermatogonium in becoming a mature spermatocyte increases 

 greatly in size and undergoes a series of important changes in which 

 several stages may.be recognized. If the duration of one of these 

 stages is measured by the number of cells in that state, it would seem 

 that a greater part of the developmental period of a primary 

 spermatocyte is passed in the spireme condition, for most of the 

 primary spermatocytes have the chromatin in the form of a spireme 

 in various stages of development. The behavior of the spireme during 

 synapsis, growth, and the formation of the chromosomes will be left 

 for a more complete description of the whole process of spermato- 

 genesis. Suffice it to say that the spireme segments and the chromo- 

 somes form against the nuclear membrane. 



Soon after their formation, the chromosomes become ari'anged at 

 the equator of the spindle with their long axes parallel to the long 

 axis of the spindle. When examined in a polar view they are seen to 

 be connected by tlireads taking the stain very similarly to the chroma- 

 tin itself (pi. 30, fig. 1). This condition has not been heretofore 

 described in mammals. The chromosomes vary somewhat in size, but 

 usually there is one considerably larger than the others. "Whether 

 this is the accessory chromosome or not has not yet been determined, 

 but one is led to that belief since often a large, round chromosome may 

 be seen when the spindle is examined in side view. 



The chromosomes vary in shape as well as in size, as shown in side 

 views of the primary spermatocyte spindles (pi. 30, figs. 3 and 5). 

 Moreover, they have a fairly constant structure resembling very closely 

 the chromosomes of the egg as described by Long and Mark (1911), 

 and by Kirkham (1907ff, b). Those that can be seen clearly, exhibit 

 a lighter area running longitudinally, and at the middle of the sides 

 darker, knob-like projections (pi. 30, fig. 5). These knobs are quite 

 prominent on the large chrosomes in figure 3. 



