10 GENERAL EMBRYOLOGY. 
The phenomena of the prophase commence with the division of the centrosome 
and attraction sphere into two parts which travel to opposite poles of the nucleus. 
At the same time the reticulum of the nucleus disappears, and in its place a con- 
voluted cord of chromatin, the skein or spirem, is formed (Fig. 3, A, B, and C); 
this is afterwards broken up into a number of segments which may be mere rods, 
but which more frequently have the form of V- -shaped loops (Fig. 3, D). The 
nucleoli disappear, and some of the filaments which radiate from the ate -formed 
attraction spheres seem to penetrate the nuclear membrane at the poles of the 
nucleus. The nuclear membrane subsequently disappears, and the filaments 
passing from the attraction spheres into the nucleus form two cones, the bases of 
which meet at the equator of the nucleus, where they fuse together, forming an 
achromatic spindle which extends between the two attraction spheres (Fis ig. 3, E). 
The loops, or rods, of chromatin are gradually grouped at the equator of the 
spindle, each rod, or chromosome, being apparently connected with one of the 
achromatic fibrils; and the prophase is completed. 
In the metaphase each chromosome is split into two halves—daughter chromo- 
somes—which separate from one another; the separation commences at the apex of 
each V-shaped chromosome, which appears to be attached to an achromatic fibril 
(@igs3; F). 
In the anaphase the daughter chromosomes pass to the opposite poles of the 
spindle. It is suggested that this is brought about by the contraction of the 
spindle fibrils, but this is doubtful, and it is noteworthy that in some cases fine 
achromatic fibrils connecting the separated daughter chromosomes are present 
(Fig. 3,G and H). Slightly before, or simultaneously with, the completion of the 
anaphase the cell body is equatorially constricted. 
During the telophase the constriction deepens and the cell is divided into two 
daughter cells. Whilst this division is taking place the daughter chromosomes, 
which are grouped in the neighbourhood of each attraction sphere at opposite ends 
of the spindle, unite into a convoluted cord, round which a nuclear membrane is 
formed, whilst the cord is converted into a reticulum, and nucleoli appear (Fig. 3, 
J, Kk, L). Therefore when the separation of the daughter cells is completed, at the 
end of the telophase, each possesses all the characteristic features of the mother 
cell. 
Reproductive Cells.—The germinal elements, the union of which is essential 
to the formation of a new being, are the ovum or female element, and the sperma- 
tozoon or male element. 
THE OVUM. 
Structurally an ovum presents all the characteristic features of a typical cell. 
It is pecular because of the large size of the nucleus and nucleolus and in the 
possession of two investing membranes, an inner one, the vitelline membrane, 
which corresponds to the cell wall, and an outer one, the oolemma or zona pellucida. 
Moreover, the nucleus always occupies an excentric position in the cytoplasm, and 
the cell body contains nutritive material in the form of yolk granules. 
The constituent parts of an ovum have received distinctive names, however ; 
thus the cell body is known as the yolk or vitellus, the nucleus is termed the 
germinal vesicle, and the true nucleolus is called the germinal spot. 
Vitellus or Yolk.—The body of the ovum, consisting as in an ordinary cell of 
cytoplasm resolvable into reticulum and sap, contains also numerous granules of 
small but varying size called yolk granules. These are highly refractile, fatty, and 
albuminoid bodies containing phosphorus and mineral salts; collectively they 
constitute the deutoplasm or nutritive yolk, in contradistinction to the cytoplasm or 
formative yolk. 
Nutritive or food yolk plays an important part in development. In some 
animals it is the only means of support for the embryo whilst in the ovum ; in most 
mammals, on the other hand, the embryo is supplied almost from the first with 
food not from the egg itself, but directly from the mother through the placenta. 
The amount of deutoplasm present in the ova of different animals therefore varies 
ereatly. 
