12 GENERAL EMBRYOLOGY. 
regarded as a secondary membrane, and is altogether different from the vitelline 
membrane. The perforations in the zona serve for the passage of nutritive 
material to the ovum; they also allow the spermatozoon to reach the ovicell. 
When the ovum leaves the Graafian follicle it is surrounded by several layers 
of cells, the innermost of which are columnar. They are derived from the cells of 
the follicle, and collectively constitute the corona radiata; the cells gradually 
diminish in size, and ultimately disappear. Their function is unknown, but 
between them and the zona pellucida there is a layer of granular matter, probably 
formed by the cells of the corona radiata, which rapidly swells up when the ovum 
is liberated from the follicle, and forms a gelatinous elastic layer called the albumen ; 
this increases in thickness as the ovum passes along the oviduct, and persists for 
some time after it enters the uterime cavity. The function of the albumen has not 
been definitely ascertained; it may act merely as a protective covering against 
undue pressure, possibly it may be nutritive, whilst in the dog it apparently helps 
to fix the ovum to the wall of the uterus. It has not been found in all mammalian 
ova, and it has not been seen round the human ovum; still it may be present, for 
human ova at the stage when it might be expected to develop have not yet been 
observed. 
Special Characters of the Ovum.—Though the ovum, as compared with an 
ordinary animal cell, presents no obvious structural modifications, it undoubtedly 
differs greatly in its capabilities and life-history. Unlike an ordinary cell it has 
no inherent power of division into equal parts, and it cannot divide in the usual 
manner until it has been fertilised by union with the male element; but before 
fertilisation occurs the ovum twice undergoes an unequal division during the period 
of ripening or maturation. 
Maturation of the Ovum.—Whilst in the ovary the ovum, at first small, 
Fia. 5.—THE MATURATION OF THE OvuM: ExTRUSION OF THE ‘“ POLAR BopiEs” (Diagrammatic). 
A, An ovum at the commencement of the process ; B, After the formation of the spindle. The chromosomes 
are gathered at the equator of the spindle in groups of four, 7.e. in ‘ tetrads,” each of which consists of 
two “dyads.” C, One apex of the spindle has projected into a bud on the surface, and the dyads have 
passed to the poles; D, The separation of the first polar body ; E, The commencement of the second 
polar body ; F, The completion of the second polar body. 
gradually increases in size. Before, or immediately after, its discharge from the 
Graafian follicle it matures in preparation for fertilisation. 
The phenomena observable during the period of maturation are essentially 
