Up to this point the object of the writer has been to convey to the reader some idea of the very interesting subject of embryonic development from the mature ovum, which is a mere speck about the 1/1000th part of an inch in diameter, to the point at which the rudiments of the young animal are formed, and the embryo is in vascular connection with the mother by the contact of the vessels of the chorion surrounding the ovum with those of the lining membrane of the uterus, so arranged that nutriment may be transferred from the parent to the offspring, and the oxygenation of the blood be effected by the process of osmosis. Thus the placental union may be looked upon as representing an organ of respiration as well as of nutrition.

Growth and development continue from the stage at which the embryo was left at the seventh week to the period when the embryo becomes the foetus, about the fourth month (fig. 542) all the organs then being miniature representations of those of the animal when separated from the mother by the act of parturition.

Fig. 542. - Foal about Fourth Month.

A detailed account of every step in the further development of the embryo would occupy more space than can well be allotted to the subject; it will, therefore, be necessary to condense the description as far as possible.