THE PHYSICAL BASIS OF HEREDITY. 175 
generates. A few moments after the sperm has entered 
a system of radiations appears around the middle piece, 
which develops into an aster surrounding the centrosome 
of the sperm (Fig. 10, &). The sperm nucleus swells 
up and rapidly increases in size, its chromatin changing 
from the compact condition in which it is arranged 
in the sperm head to a reticulate condition (Fig. 10, C). 
The chromatin reticulum of the egg nucleus becomes 
also more clearly visible. Sperm aster and sperm nu- 
cleus now move in toward the egg nucleus, the aster 
usually preceding. As the nuclei approach the sperm 
nucleus increases still more in size until it becomes in- 
distinguishable from the egg nucleus (Fig. 10, C). The 
chromatin network of each now breaks up into a number 
of chromosomes, one half of the number found in the 
somatic cells, and the nuclei come into contact, fusing 
together in some cases. In the sea urchin, Zchinus, the 
number of chromosomes is eighteen, nine would there- 
fore be found inthe germ nuclei; for the sake of clear- 
ness and simplicity but two are represented in the dia- 
gram, those of the sperm nucleus being slightly shaded 
while those of the egg nucleus are black. The centro- 
some divides together with its aster 
(Fig. 10, D), the two daughter centro- 
somes move apart to opposite poles of the egg, and the 
typical amphaster of cell division is formed (Fig. 10, Z), 
the nuclear membranes disappearing and the chromo- 
somes being drawn together into the equatorial plate 
where each splits longitudinally. The halves are drawn 
by the mantle fibrils toward the opposite poles and the 
egg divides transversely into two cells (Fig. 10, /). This 
process of division is repeated continuously in each of 
the resulting generations of cells, and from the mass 
‘of cells thus formed develops the new organism. Each 
cell in the two-celled stage has received half of its 
13 
Cleavage. 
