OVUM STAGE. There are 110 observations on normal ova of the first nine or ten days. It is evident from the material of the later part of the ovum stage that there is an early and pre cocious development of the chorion and villi.
T h c youngest known 110 r in al ovum was de scribed by Peters in 1899. It is ten to eleven days old and consists of a vesicle 3 X X 1.5 min. in size. (Fig. 1.) The vesicle is formed by the ellorionie m e consisting, of an outer layer of epithelial cells covered by nu merous villi which are in con tact with the uterine wall of the mother, and an inner layer of inesenellyina. Attached to this inner layer at one side is the small embryo but .19 nun. in length. lt is apparently simple in structure, consist ing, of an epithelial plate facing the small amniotic cavity, On the other side of the plate is a layer of mesen eliyma, and projecting from this is the yolk-sac lined by entodermal cells. The projecting embryo is surrounded by mesenchyma continuous with that lining the ehorion. The epithelial plate of the embryo and the epithelium of the amnion were probably at an earlier stage continuous with the epithelium of the chorion and subsequently emit off by sinking down into the vesicular cavity. Already, then, in the youngest known ovum the so-called three primary germ-layers are present. (See EmBRYOLOGY). From the epithelial layer develop the epidermis of the skin and its ap pendages (hairs, nails. sweat-glands, etc.), the central nervous system, and portions of the eye and ear, month and nose. From the middle or inesenchymal layer develop the skeletal. muscu lar. circulatory and uro-genital systems: and lastly from the inner or entodermal layer, here represented by the lining of the yolk-sae. de velop the alimentary tract. trachea, and lungs, liver and pancreas. and bladder.
The next important human ovum described contained an embryo .37 min. in length, and about eleven days old, attached at one side by a broad pellicle. Along the centre of the epithelial plate i, mu slight grome. the first Irace of the ccuilal canal of t 1 II • M.ryons system; and
it small diverticidlim of the sae (the allantois) projects into the pellicle. (Fig. day later the circulatory system has begun to develop, as a simple tubular heart lying in the mesen chyme between the head end of the em bryo and the yolk-sac. From the head end of the heart is given off the aorta, which divides immediately into three pairs of arches that pass around the primitive foregut to unite beneath the medullary plate into the dorsal aorta. Blood-vessels extend between the embryo and chorion. The yolk-sac has now be come somewhat constricted along its attachment to the embryo, leaving pockets of the entoderm under the head and tail ends of the epithelial plate, which form the primitive foregut and hind gut. The medullary folds, which are the first rudiments of the central nervous system, are now well marked, as also is the groove lying between them. Just beneath the epithelial plate, and lateral to the medullary folds, the mesen chyma is divided into thirteen pairs of segments, the first differentiation for the muscular system.
At the age of two weeks many important changes have taken place. The embryo has greatly increased in length and is curved into a semicircular form. (Fig. 3.) The medullary plate is converted into a thick-walled tube, and the groove into its canal. At the anterior end of this tube are enlargements marking the begin nings of the brain. while the remainder of the tube forms the spinal cord. From the foregut has arisen the pharynx with two gill-pockets, and a thyroid pocket, but the main portion of the alimentary tract is still embodied in the large yolk-sae. The forebrain of an embryo about 14 days old has a marked ventral bend, and between the forebrain and the heart is an invaginat ion of the skin to form the mouth-cavity, not yet con nected with the pharynx. Projecting from the forebrain are the optic vesicles and farther back ace invaginations of the ectoderm for the internal ear. Trio gill-clefts and three branchial arches are present branches of the aorta pass through the latter.