246 GERM-CELL CYCLE IN ANIMALS 
of the germ-cell cycle (Chapter II), with the par- 
thenogenetic or fertilized egg after the maturation 
processes have been completed, and to exclude all 
references to the accessory chromosome until later. 
It may be pointed out first that the number of 
chromosomes in the cells of any individual of a 
species is, with few exceptions, constant. Thus the 
thread worm of the ‘horse, Ascaris megalocephala 
var. univalens, has two; <A. megalocephala var. 
bivalens, four; the nematod, Coronilla, eight; the 
mole cricket, Gryllotalpa vulgaris, twelve; the 
bug, Pentatoma, fourteen; the rat, sixteen; the 
sea urchin, Echinus, eighteen; the salamander, 
Salamandra maculosa, twenty-four; the slug, Lomax 
agrestis, thirty-two; and the brine shrimp, Artemia, 
one hundred and sixty-eight. This number, however, 
is reduced one-half during the maturation of the 
eggs and spermatozoa so that the mature eggs and 
spermatozoa possess only half as many chromosomes 
as the other cells in the body; for example, the body 
cells, odgonia, and spermatogonia of the rat are 
provided each with sixteen chromosomes, but the 
mature eggs and spermatozoa contain only eight. 
Parthenogenetic eggs differ from those that require 
fertilization, since in these the complete or diploid 
number of chromosomes is retained. When cleavage 
is inaugurated in such eggs, a spindle is formed, the 
chromosomes are halved, and each daughter cell 
acquires one-half of each chromosome as in ordinary 
mitosis. In fertilized eggs, however, the nucleus 
brought in by the spermatozodn fuses more or less 
