EMBRYOLOGY. 30 On the other side of the plate is a layer of mesen- chvma, and projecting from this is the yolk-sac lined by entodermal tells. The projecting embryo is surrounded by mesenehyma continuous with that lining the chorion. 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 EMBRYOLOGY. FlG. '2. llEGINNING OF EMBRYO. Median 6ection of a model of an embr.vo, 1.54 mm. long: bjd, amnion ; :un'. amniotic cavity; a/, allantois; /-, pedicle; c, ehordu dorsalis; cb, chorion: en, neurenteric canal; d, yolk-sac; >lh. itscavitj; g, £,£■, blood islands; b. place where heart will develop. (Alter Spee. i cut off by sinking down into the vesicular eai ity. Already, then, in the youngi -t known ovum the so-called three primary genu layers are present. (See Embryology). From the epithelial layer develop the epidermis of the skin and ils ap- pendages (hairs, nails, sweat-glands, etc.), the l nervous system, and portions of the eye and ear, mouth and nose. From the middle or mesenchymal layer develop the skeletal, muscu lar. circulatory, and uro-genital systems; and lastly from the inner or entodermal layer, here ented by the lining of the yolk sac, dc velop the alimentary tract, trachea, and lungs, liver and pancreas, and bU The next important human ovum described contained an embryo .37 mm. in length, and about eleven d» al one .-ide by a AN BABI/Y I.MMHYO. ■ <>f 2.1 mm. loner: .ion; all, allantois; hi*, hr". Drat and I nd gul m (AfOT broad I be cent re ..f the epil helial Lhe first I race of the i ■ rvou« j ir in ; and 'i diverticulum le allantois) o. dicli . ' Fig. '. i daj latci 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- ehyma 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. 1 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 i- si ill embodied in the large yolk-sac. The forebrain of an embryo about 14 days old has a marked ventral bend, and between the forebrain and the heart is an invagination of the skin to form the mouth-cavity, not yet con- nected with the pharynx. Projecting from the brain are the optic vesicles and farther back are invaginations of the ectoderm for tin' internal ear. Two gill-clefts and three branchial arches present; branches of the aorta pass through the latter. Embbyok With the formation of the medullary tube and its cephalic enlargements begins the embryonal stage. During the third week the embryo grows rapidly in size and attains a length of aboul four millimeters. The brain increases in size and shows the three primary di- ms. The optic and otic vesicles become more inent. Two more gill clefts and three more gill-arches appear, caudal to the ones already formed. The attached area of the yolk-sac has diminished. The mouth em ity communicates with the pharynx. l' the twenty-first day the limb-buds appear. Internally the heart is en- larged and takes the form of an S-shaped tube. From the aorta novi ari . live pairs of arteries Which pass through the five pairs of gill-arches joining on the of the pharynx into a COmi i i narrowing of the attachment of the yolk sac more of the primitive gui has been folded off, so that now the foregut, midgut, and hindgut an list inguished. In the foregut can alreadj be made oul the pharynx with iis diverticula, the - and the stom- ach. Of the phai I divert icula, there arc oi responding to the on the extei na I md separated bj i bi im i'i urn the I'ha i Iso tarted the divert iculum for the resp 'i lung I lie Eorma the mall mil portion of the large in-
