After puberty, at intervals averaging about four weeks, the genital organs of the female become congested, and at the same time a Graafian follicle is ruptured and its contained ovum set free. Coincidently with the rupture of the follicle, the fimbriated extremity of the Fallopian tube becomes closely approximated to the spot where the follicle lies, so that the ovum, instead of falling into the abdominal cavity, passes into the canal of the Fallopian tube, down which it is conveyed to the uterus.

The usual place for the ovum to meet the spermatozoa, and to be impregnated, is the Fallopian tube.

When the ovum reaches the uterus, if it be unimpregnated, it is cast out with the surface cells of the mucous membrane of the uterus, which are destroyed, and escape along with a sanious fluid. The whole of these phenomena constitute a menstrual act.

If, however, the ovum becomes impregnated, it remains in the Fallopian tube some days, during which time the mucous membrane of the uterus becomes so hypertrophied as to retain the ovum when it reaches that organ.

The human ovum is a cell consisting of a mass of protoplasm enclosing a nucleus and nucleolus, and surrounded by a cell wall. On its outer surface is an irregular layer of cells, the remains of that part of the tunica granulosa which encircled the ovum in the Graafian follicle. The cell wall of the ovum is called the vitelline membrane or zona pellucida, and the mass of granular protoplasm it encircles, the vitellus or yolk, and in this is a nucleus - the germinal vesicle, which contains a nucleolus - the germinal spot.

Beneath the outer covering of calcareous material of the hen's egg there is a white membrane, which encloses a transparent albuminous substance known as the white of egg. Inside this is a yellow fluid mass, the yolk, surrounded by a delicate membrane, vitelline membrane. The yolk is made up of two varieties of material of different shades of color, the white and the yellow yolk. Of these the yellow forms the greater part, the white being arranged in thin layers, which separate the yellow yolk into strata. In the centre of the yolk it forms a flask-shaped mass, with its neck turned to the upper surface, upon which a portion of the yolk rests called the cicatricula. This cicatricula, which lies between the vitelline membrane and the white yolk, is the active growing part of the egg, and out of it is developed the chick and the embryonic membranes.

Extending through the albumin from the vitelline membrane to the ends of the eggs are two twisted membranous cords - the chalazce, which fix and protect the delicate yolk from shocks, but allow it to rotate, so that the cicatricula is always the uppermost part of the yolk when the egg is on its side.

The main structural differences between the human ovum and that of a fowl are apparent from the above description: the essential peculiarity of the development of the hen's egg is that only a portion of the yolk is engaged in the formation of the first signs of the chick and its membranes, by far the greater part of the egg, both yolk and albumin, being utilized as nourishment during the subsequent stages of development.

Ovum. {Robin).

Fig. 267. Ovum. {Robin).

a. Zona pellucida or vitelline membrane.

b. Yolk.

c. Germinal vesicle or nucleus.

d. Germinal spot or nucleolus.

e. Interval left by the retraction of the vitellus from the zona pellucida.

After the egg has been laid, it obtains no help from the outside world, except the oxygen of the air and the heat of the mother's body; it is, as it were, fenced in with a protecting membrane, garrisoned with the quantity of provisions required, and by the warmth of the hen's body stimulated to growth and activity.

The whole of the human ovum, on the other hand, undergoes segmentation and differentiation in the primary formation of the embryo, which subsequently is supplied with the necessary nour-ishment from the maternal circulation. The life and growth of the human embryo depend upon supplies from the mother, the ovum not having within itself any store of nutrient material.

Diagram of a section of an unimpregnated fowl's egg.

Fig.268. Diagram of a section of an unimpregnated fowl's egg. (From Foster and Balfour, after Allen Thomson).

b1. Blastoderm or cicatricula.

w.y. White yolk.

y.y. Yellow yolk.

ch.l. Chalaza.

i.s.m. Inner layer of shell membrane.

j m. Outer layer of shell membrane.

s. Shell. a.ch. Air space. iv. The white of the egg. vt. Vitelline membrane. x. The denser albuminous layer which lies next to the vitelline membrane.