FOURTH WEEK. During the fourth week growth is relatively more active than at any other time. The embryo about doubles in length. During the first part of the week the embryo becomes very much flexed, so much so that the head and tail nearly touch. The brain-vesieles are better de veloped, as also are the gill-clefts and arches, the eyes, cars, and nasal pits. The S-shaped tubular heart, with its single cavity is rapidly changing into a complicated four-chambered structure with imperfect partitions. By the end of the fourth week (Fig. 4), the anterior enlarged portion of the head has become bent at right angles to the main axis of the body. From the first gill-arch a maxillary process is developing which will ultimately form .a portion of the upper jaw; the main portion of the arch will form the lower jaw. The limb-buds have in creased in size. The arm-bud projects front the cervical region, :mud the leg-bud from the lumbar region. The yolk-sac is very small and the amniotic sac is much enlarged. Internally the nerves have begun to grow from the medullary tube or central nervous system into the mesen chyma. Plexuses are forming, but the nerves are
not attached to their end-organs. There are now 33 primitive segments formed from the mesen ehyma lateral to the spinal axis. Aluscle-fibres are beginning to differentiate from the cells of these segments, and their ventral ends grow into the primitive abdominal wall formed by the grow ing around of the amnion. Condensed mesenchyma marks portions of the vertebral column. In the arm-bud the mescnchyma is beginning to differ entiate into skeletal and premusele tissue. The Wolffian ridge has increased in size, and the Mfillerian duct is now formed and runs parallel with the Wolffian duct. A _ _ diverticulum from the lower end of the Woltlian duet in dicates the beginning of the permanent kidney and its duct. On the Wolffian ridge are also seen the first traces of the sexual glands,' but not until the fifth week can sex be determined, even by micro scopical examination.