THE OVUM. Tt 
Ova in which there is no deutoplasm are spoken of as alecithal. Such ova, if 
they exist, are very rare; most of those usually classed under this head undoubtedly 
contain a certain amount of deutoplasm granules scattered throughout the cyto- 
plasm, and are better described by the 
term oligolecithal. The size of an ovum 
is determined by the amount of food yolk 
present, and all oligolecithal ova are small ; 
the human ovum, which may be taken as 
a type of the class, is about ‘2 mm., or 
zith of an inch in diameter. 
As the deutoplasm is increased in 
amount the ovum is increased in size. 
The deutoplasm also tends to accumulate 
‘In certain situations; if the accumulation 
is at one extremity of the cell the ovum 
is described as telolecithal ; such ova are 
naturally divisible into two areas or poles, 
a cytoplasmic or formative pole, and a 
deutoplasmic or nutritive pole. 
In eutelolecithal ova the deutoplasm 
almost entirely displaces the cytoplasm 
from one pole, as in the egg of the fowl, 
in which the cytoplasm is represented by 
a disc spread over one pole of a large 
Fic, 4.—THe OvuUM AND ITs COVERINGS 
(Diagrammatic), 
The corona radiata, which completely surrounds the 
deutoplasmic mass. In many of the ovum, is only represented in the lower part of 
arthropoda the deutoplasm accumulates the Henne: 
A x Saas 5 1. Corona radiata. 5. Vitellus or Yolk. 
at the centre of the OV um, which is there- 2. Granular layer. 6. Germinal vesicle (nucleus). 
fore termed centrolecithal. 3. Vitelline membrane. 7. Germinal spot (nucleolus). 
4. Zona pellucida (oolemma). 8. Nuclear membrane. 
The germinal vesicle or nucleus of 
the human ovum is about ‘(05 mm. or 33,th of an inch in diameter, ze. 4 the 
size of the whole ovum. It lies excentrically in the yolk, and has the usual 
characters of a cell nucleus, z.e. it possesses a nuclear membrane within which is 
the karyoplasm, divisible into reticulum or nucleoplasm, and nuclear juice. The 
nucleoplasm consists of chromatin and achromatic fibres (linin), and the nuclear 
juice contains one or more spherical and highly refractile true nucleoli or 
germinal spots; the nodes of the reticulum constitute false nucleoli. 
In addition to the nucleus, the vitellus, at certain periods, also contains a structure 
known as the vitelline body or body of Balbiani. This body is a spherical structure 
which appears when the primordial ova cease to multiply and begin to increase in size. 
Apparently it is derived from the nucleus, and it consists of a central nodule surrounded 
by a zone of modified protoplasm. It disappears long before the ovum becomes mature, 
and it probably represents an ancestral organ corresponding with a portion of the macro- 
nucleus of the infusorian ovum. 
The Vitelline Membrane.—The vitelline membrane is simply the peripheral 
portion of the vitellus, modified and transformed into a fine structureless envelope 
which covers the outer surface of the yolk. It is usually closely applied to the 
inner aspect of the outer membrane, the zona pellucida, and is best seen in the 
dead ovum and after treatment by reagents. It is therefore thought by some to 
be merely a condensation of the outer part of the vitellus produced by the action of 
the reagents. There is evidence, however, to show that it is present in the normal 
living ovum. 
The Zona Pellucida or Oolemma.—This membrane is thick, tough, and refrac- 
tile. It serves as a protective covering for the ovum, and persists for a considerable 
time after its fertilisation, only disappearing when the ovum becomes attached to 
the uterus. It is perforated by numerous fine canals, which give to the broad clear 
membrane a finely striated appearance, from which circumstance it has been called 
the “zona striata.” The zona pellucida is not formed by the ovum, but is secreted 
by the cells of the Graafian follicle in which the ovum les; it is consequently 
