ExtralYONAL Sr cr. With the formation of the medullary tube and its eephalie enlargements begins the embryonal stage. During, the third week the embryo grows rapidly in size and attains ix length of about four millimeters. The brain inereases in size and shows the three primary di visions. The optic and otic vesieles become more prominent. Two more gill-clefts and three more gill-a•ebes appear. caudal to the ones :Already formed. The attached area of the yolk-sae lids diminished. The mouth-eavity communicates the pharynx. By day the limb•buds appear. Internally the heart is en larged and takes the form of an S-shaped tulle.
From I he aorta now arise five pairs of arteries which pass through the live pairs of gill-arches joining on the dorsal side of the pharynx into a common dorsal aorta. Br the narrowing of the attaehment of the yolk-sac inure of the primitive gut has been folded off, so that now the foregut, inidgut, and hindgut are to be distinguished. In the foregut van already be made out the pharynx with its ilivertieula, the wsophagus and the stom ach. Of the pharyngeal divertienla, there are
four pairs of gill-pockets, corresponding to the gill clefts on the external surface, and separated from the gill-elefts by a membrane. From the pharynx has also started the divert ienlum for the reviralory Inke lungs. The midget forms the small intestine and portion of the large in testine, and from it have arisen diverticula which give rise to the liver and pancreas. The hindg,ut gives rise to the rest of the large intestine and the rectum. By enlargement and growth of the amnion around the embryo until it joins the pedicle has been formed the body-cavity. From this later are separated the pleural, pericardial, and peritoneal cavities. The primitive uro-geni tal system has appeared as a ridge along the dorsal wall of the body-eavity at either side of the median line in the posterior half of the embryo. This ridge contains the Wolffian duct and tubules and the rudiment of the Aliillerian duct. There is no sex differentiation.