THE PHYSICAL BASIS OF HEREDITY. 179 
in or near the last two divisions of the germ cells pre- 
vious to their fusion—that is, in the egg—in the di- 
visions forming the polar bodies, and in the sperm, in 
the last two divisions of the spermatocyte which pro- 
duce the four spermatids out of which develop as many 
mature spermatozoa. The phenomenaare exactly homol- 
ogous in both cases, as has already been pointed out, 
differing only in the minor details which do not affect 
the end result. Two peculiar features mark these di- 
visions off from all the others which precede and follow 
them. One of these is the absence of an intermediate 
resting stage between them, the second division follow- 
ing immediately upon the first without the reconstitu- 
tion of the chromosomes into the skein stages. The 
second peculiarity lies in the fact that the chromatin 
masses (not the individual chromosomes) appear in one 
half the typical number of the chromosomes in the first 
division, and are usually arranged in “tetrads,” or 
groups of four rounded, deeply staining bodies connected 
by linin fibres. These tetrads are always one half the 
number of the original rod or thread-like chromosomes. 
Thus in Fig. 11, 4 represents a sperma- 
pres togonium nucleus of Ascaris with the 
scaris. A a 
7 four chromosomes, showing the longi- 
tudinal splitting preparatory to division. Fig. 11, 2, rep- 
resents an early spindle stage in the division of the 
primary spermatocyte, in which not four band-like 
chromosomes, but ‘wo tetrads, or chromatin groups of 
four rounded bodies are found. Fig. 11, C to /, show 
clearly the further steps in the spermatogenesis. In Fig. 
11, C, the tetrads are grouped in the equatorial plate, 
and in Fig. 11, D, in the closing stages of the first divi- 
sion into two spermatocytes, each tetrad has divided into 
two “dyads,” which are drawn to the poles, and the 
division of the cell body follows. Without an interven- 
Reduction in 
