THE ORGANIC CELL 
81 
the periphery can be seen spermatozoa trying to get into the egg 
substance ; at the right-hand side one has been more success- 
ful than the rest, having pierced the peripheral envelope, 
and is passing into the egg-cytoplasm. As soon as the head 
of one spermatozoon has entered, a new membrane forms 
around the egg substance which prevents the entrance of any 
more. There is evidence of a definite attraction between 
the germ-cells. The nature of the attraction appears to be 
chemical, since the spermatozoids of ferns are actively attracted 
by solutions of malic acid ; those of mosses are not affected 
by malic acid, but by cane-sugar. This attractive force is 
not inherent in the nucleus alone, but is also present in the 
cytoplasm. The head and middle piece pass into the egg 
substance, the tail remaining in the egg membrane, where it 
degenerates. Very shortly after the entrance of the sperm, 
a series of radiations make their appearance around the middle 
piece, forming an aster surrounding a centrosome, B. The 
head of the spermatozoon or sperm nucleus swells, increasing 
to a considerable size, its chromatin becoming arranged to 
form a reticulum, C. At the same time the chromatin reticulum 
of the egg nucleus becomes more definite. Sperm aster and 
sperm nucleus now move toward the egg nucleus, the aster 
generally leading the way. On nearer approach the sperm 
nucleus increases still more in size, until it becomes indis- 
tinguishable from the egg nucleus C. The chromatin network 
of each nucleus now forms a number of chromosomes (one half 
the number in each nucleus as are found in the somatic cells). 
The nuclei come together and fuse. 
In the sea-urchin Echinus the number of chromosomes 
is eighteen, nine being found in each germ nucleus. In the 
diagram, for the sake of simplicity, only two are shown, those 
of the sperm being shaded, while those of the egg nucleus 
are black. 
The centrosome divides with its aster (D), the daughter 
centrosomes moving apart to the opposite poles of the egg, 
thus forming the usual amphiaster of cell-division (E) ; the 
chromosomes become arranged in the equatorial plane of 
the spindle, and each one divides longitudinally. The halves 
are now drawn apart by the astral rays towards the opposite 
