46 GERM-CELL CYCLE IN ANIMALS 
primary spermatocyte is seen to be divided into 
four parts, thus forming in this case two tetrads (C). 
During the division of the primary spermatocyte, 
as shown in D, E, and F, half of each tetrad, or two 
dyads, passes to each daughter cell. The division 
of the daughter cells, which are known as secondary 
spermatocytes (G H), results in the separation of the 
two parts of each dyad so that each of the four 
spermatids (H):. receives one member of each original 
tetrad or two monads. Thus the chromosomes 
(monads) of the spermatids (H) are already formed 
in the primary spermatocytes (C) by two divisions ; 
whereas the nuclear and cell divisions do not occur 
until later. The spermatids (H), which proceed 
to metamorphose into spermatozoa, possess, there- 
fore, only two chromosomes, 7.e., one-half of the 
number present in the spermatogonia (A) and so- 
matic cells. 
Tetrad formation does not occur in most animals; 
but usually the members of the bivalent chromosomes 
become separated on the first maturation spindle, 
the pairs appearing U-, V-, or ring-shaped, as in 
Fig. 62. Each secondary spermatocyte receives 
one-half of each haploid, bivalent chromosome. The 
second maturation mitosis then ensues, during which 
each daughter cell is provided with one-half of each 
chromosome as in ordinary mitotic division. Be- 
cause of the peculiar behavior of the chromosomes 
the first division is often called the heterotype, 
whereas the second is known as the homotype divi- 
sion, The final results are the same whether tetrads 
