The spermatogenesis of Hydra. 407 
was only when the transition stages of Fig. 22—26 were discovered 
that it seemed they could be identical. This longitudinal splitting 
of the V-shaped chromosomes and the subsequent fusion of the 
approximated arms produces an apparent transverse division and 
makes an equation division appear very like a reduction division. 
Since we are sure that no reduction division occurs in some sper- 
matogonia of the first order, we conclude that it does not occur in 
any in spite of the apparent reduction in Fig. 36. 
Reduction. The reduction in the number of chromosomes 
occurs in the telephase of the first spermatogonic division. The 
chromosomes of the spermatocyte of the first order, though showing 
no signs of tetrad structure are the equivalents of the tetrads: 
Their first division is a longitudinal division as each daughter 
chromosome contains four chromomeres and the spireme prior to the 
division of the spermatocytes of the second order with spermatids 
contains twenty-four chromomeres. Since the chromomeres from 
which each of these was derived were bivalent and the division is 
not a reduction division, the chromomeres of the spermatocytes of 
the second order are bivalent. 
In the division of the spermatocytes of the second order there 
probably occurs a reduction division for each daughter chromosome 
contains two chromomeres (Fig. 18). This might be explained as 
an equation division however, similar to that of the spermatogonia 
(Fig. 36). But the contrast to the division of the spermatocyte of the 
first order where there are four daughter chromomeres in the daughter 
chromosomes would seem to indicate that the division of the chromo- 
somes of the spermatocytes of the second order is at right angles to the 
division in the first order of spermatocytes, so that the latter being an 
equation division the former is a reduction division. The chromo- 
meres which appear in the head of the sperm are at any rate bivalent. 
Changes in volume during mitosis. 
Throughout the division stages of spermatogonia and spermato- 
cytes there are constant changes in both absolute and relative sizes 
of cells and nuclei. There is a slight decrease in the size of the 
cell throughout the prophase up to the formation of the spireme. 
The nucleus meantime is increasing in size. After the spireme forms 
the cell begins to grow and attains its maximum size during the 
early anaphase. Immediately after division the nucleus is larger, 
