In making a study of the entire embryo we note there has been a rapid growth of the dorsal surface of the head, which has become more bent. This bend, in the region of the midbrain is called the cephalic flexure. The forebrain and part of the midbrain form almost a right angle with the rest of the head. The head is compressed laterally and free from the blastoderm. The dorsal side of the trunk is turned up, and there is a twisting of the axis of the embryo just back of the heart. The tail fold begins at about this time and may or may not be visible. The optic vesicles are relatively smaller in relation to the brain than in 1. Note the part of the forebrain to which they are attached. Observe the inner and outer layers of the retina, the lens, the choroid fissure, and the cavity of the vitreous humor. Note the auditory vesicles and whether or not they are closed sacs. Note that the heart has grown in length and has become doubled on itself. The two ends are fixed. Note the relation of the heart to the afferent and efferent blood-vessels. Note that two, and possibly three, visceral pounches are visible. Note whether or not they are ventral to the midbrain. The first, the hyomandibular pouch, is bounded in front by the first visceral, or mandibular, and behind by the second visceral, arch. The second pouch is bound in front by the hyoid, and behind by the third visceral arch. The third pouch is bounded in front by the third visceral arch, and behind by the fourth. Note the number of mesoblastic somites and the condition of the mesoblastic segmental plates. Note how far back the foregut is closed. Locate the head fold of the amnion, and note how far back it is closed. Note changes that have taken place in the vascular area. In studying the sections it will be found that a section cut transversely to the trunk will pass horizontally through the forebrain and through the midbrain. Study a section through the trunk a short distance behind the heart. Observe the elevation of the axis of the body. Note the way in which the lateral folds, or the lateral limiting sulci, in the somatopleure, delimit the embryonic from the extra-embryonic area. Note the appearance of the mesenchyme, the approximation of the two dorsal aortæ, the appearance of the amniotic folds. Observe in the mesoderm the posterior cardiac veins, and the myotomes, or muscle plates. The sclerotome is made up of the mass of mesenchyme between the myotome, on the one hand, and the neural tube and the notochord, on the other. Note the folding of the splanchno-pleure, and note if there is present the Wolffian duct, or the nephrotome. Study sections through the optic vesicles. Note if there is the beginning of the lens. Do you note the diverticula of the pharynx? Can you identify the closed amnion and the chorion?