194 CULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The spermatocytes are represented in Plates 1 and 2, Figs. 4-47. 
I have seen no evidence of a longitudinal splitting of the chromatic 
thread in the prophases of the first division. I still believe that there 
are two modes of formation of “ Vierergruppen” (compare Figs. 4 and 
16). The centrosomes are very conspicuous in the best preserved prep- 
arations, especially during the first division of the spermatocytes (Plate 
1, Fig. 25). During the second division of the spermatocytes (Figs. 26— 
35 and 39-45) the centrosomes are not readily distinguished, partly 
because the chromatic elements are themselves spheroidal and of about 
the same size as are the centrosomes. The interzonal filaments are very 
distinct (Plate 2, Figs. 31-35), particularly after Hermann’s killing 
fluid. There are stainable particles on the fibres, and some of the fibres 
are thickened for a part of their length (Plate 2, Figs. 39, 40, 42). Fig- 
ures 39 and 40 represent equatorial views, and in such cases two or 
three thickened masses of fibres are seen, which are continuous with 
ordinary delicate filaments as they approach the chromatin. Ordinary 
filaments are also to be seen between these thickened strands. An 
oblique section of such a spindle is represented in Figure 43. Figure 
42 is from a cross section. It is to be seen from the last figure that the 
thickened fibres are arranged in four somewhat unequal groups. In an 
equatorial view (Figs. 39, 40) one or two groups are obscured by the 
others. 
I have used the term “thickened fibres.” As a matter of fact I can- 
not say that the appearance is not caused by an accumulation of stain- 
able (lanthanin) granules along that portion of the fibres. It is not 
a simple swelling of the fibres; for in that case they would not become 
darker, as they are. It is not a massing together of the fibres; for then 
there would be seen tracts free from fibres, which is not true. Cross 
sections of the fibres are easily seen, being of considerable size; but they 
are not to be confounded with the chromosomes, which have a different 
structure and stain differently. There is in some cases (Fig. 39) a dark 
granular band across the middle of these thickened strands. 
This peculiar condition of the interzonal filaments shown in Figures 
39, 40, 42, was found in only a single individual. The testis was killed 
in Hermann’s fluid, the preservation being very good. Many other sper- 
matocytes in the same testis show no thick fibres (compare Fig. 31 with 
Fig. 40). Iam therefore unable to say whether it is an unusual condi- 
tion, a variation from the ordinary process, or a regular stage which is 
so quickly passed through that it is only occasionally found. 
Spermatids immediately after the second division of the spermatocytes 
