THE SPERMATOZOON. 13 
similar to those met with in the mitotic or karyokinetic division of cells. The 
excentrically situated nucleus (germinal vesicle) moves to one pole of the ovum, 
the nuclear membrane and the nucleoli disappear, and an achromatic spindle is 
formed, but, so far as the mammalian ovum is concerned, there is no evidence 
either of centrosomes or attraction spheres at the poles of the spindle ;_ possibly 
they are present, but they are not visible under the conditions in which the ova 
can be observed. 
At first the spindle lies horizontally, but gradually it turns till its long axis is 
perpendicular to the surface of the ovum; then one pole of the spindle is protruded, 
carrying with it a small mass of the cytoplasm and forming a bud. 
Simultaneously half the chromosomes present in the nucleus disappear, and the 
remaining half accumulate at the equator of the spindle in the form of rods. Each 
rod divides into four parts and thus becomes a “ tetrad” (end of prophase). Each 
tetrad then separates into two “ dy ads” (metaphase). In the next stage (anaphase) 
the dyads diverge, one half passing to the outer pole of the spindle which projects 
into the bud, the other half passing to the inner pole. The bud with its dyads then 
separates, forming the first polar body. Thus both the first polar body and the 
remainder of the ovum, at the end of the unequal division which has taken place, 
possess the same number of chromatin particles (chromosomes) as the original ovum. 
The second unequal division of the ovum commences directly on the termination 
of the first, and results in the formation of a second polar body. The achromatic 
spindle is reformed and the remaining dyads are grouped at its equator. The 
outer extremity of the spindle projects into a second bud, whilst the two halves of 
each dyad diverge and pass to opposite poles of the spindle. Those which group 
themselves at the outer pole separate with the bud, and the second polar body is 
completed. 
The half-dyads at the inner pole of the spindle travel towards the centre of the 
ovum, and form the chromatin reticulum of a new nucleus in the cytoplasm of the 
ovum, which is known as the female pronucleus. 
Therefore the mature and the immature ova differ from each other chiefly in 
that the former contains only half the number of chromosomes present in the 
latter. 
THE SPERMATOZOON. 
Spermatozoa are modified cells produced in the testicles or male generative 
glands. They are formed by a division of the spermatogonia or sperm-mother cells, 
which correspond with primitive ova. The daughter cells of the spermatogonia 
are called spermatocysts, and the grand-daughter cells spermatids. By various 
processes, during which they fuse with special basal, sustentacular, or nurse cells, 
the spermatids are converted into spermatozoa, which probably, though this has 
not yet been proved in mammals, contain half the number of chromosomes which 
were present in the sperm-mother cells. There is no doubt that the mature ova 
and the spermatozoa, so far as their development and chromatin particles are con- 
cerned, are very similar. Each is formed from a primary germinal cell, that is, 
from the primitive ovum and sperm-mother cell respectively. Each of these 
undergoes division and the daughter cells produced, that is, the functional ovum 
and the first polar body in the former case, and spermatocysts in the latter, 
contain a number of chromosomes identical with that originally present in the 
mother cell. By a second division, which follows immediately on the first, the 
daughter cells, with the exception perhaps of the first polar body, are transformed 
into ) grand- -daughter cells. These are therefore represented by the second polar 
body and the mature ovum in the case of the female, and by the spermatids, which 
ultimately become spermatozoa, in the male. Kach grand-daughter cell contains 
only half the number of chromosomes which are originally present in the mother 
cell. But the cells derived from the primitive ovum differ from the descendants 
of the sperm-mother cell in that only one, the mature or functional ovum, appears 
to be capable of further development, whilst the four spermatozoa are apparently 
of equal value. 
