26 
W. E. AGAR. 
PI. 1, fig. 3, in wliicli it is seen that the large chromatin 
blocks are the centres for the formation of the chromosomes. 
These appear to grow out as continually lengthening bands, the 
substance of the chromatin blocks getting used up in the 
process (PI. 1, figs. 3 and 4). In fig. 4 the chromosomes are 
fairly well defined towards one pole of the nucleus, but at the 
other are not nearly so far advanced. In PI. 1, fig. 5, we 
have the fully formed spireme. It is distinctly formed of 
separate chromosomes, though the ends of these show a 
marked tendency to adhere together. In some cases they are 
connected by thin threads. It is easy to see how this condition 
could give rise to the apparently continuous spiremes, which 
have been seen in many forms. 
In PI. 1, figs. 6 and 7 are shown the equatorial plates of 
one of the largest and one of the smallest (earliest and latest) 
generations of spermatogonia. These are referred to in 
another connection later (pp. 30, 37). Judging from the size 
of the nuclei shown in PI. 1, figs. 2, 3, 4, 5, these belong to the 
middle generations of spermatogonia, and would result in an 
equatorial plate intermediate in size between those shown in 
PI. 1, figs. 6 and 7. 
The localised, circumscribed area, out of which each 
chromosome takes its origin, is strikingly different from 
what we find in the early meiotic prophase. Comparing PI. 
1, figs. 3, 4, 5 with the equatorial plates, we see that 
when first formed each chromosome is but little longer than 
it will be at the metaphase — note, for instance, the very 
small horse-shoe-shaped element in PI. 1, fig. 4. In the 
meiotic prophase, on the other hand, the chromosomes at 
their first appearance are enormously long and thin (lepto- 
tene threads). This difference in the early prophase is 
doubtless correlated with the structure of the resting nucleus. 
In the spermatogonia — at any rate those of the middle and 
later generations, to which the stages described belong — the 
chromosomes appear not to enter such a complete “ resting” 
or diffused state between the mitoses as they do during the 
growth period of the primary spermatocyte. 
