Note the form of the otocyst, or auditory sac. Note above which visceral arch it lies. Note the number of visceral clefts. The visceral arches are formed by the thickening of the walls of the bounding clefts. The visceral arches are as follows: the first is the mandibular, or hyomandibular, arch, which is in front of the first cleft. From this there is developed the lower jaw. Note if there is a maxillary process arising from the dorsal angle of the arch. The second arch is the hyoid arch, which is located behind the first cleft. Then follow in order the third, the fourth, and the fifth visceral arches. Note above the mandibular arch the rudimentary trigeminal ganglion, and above the hyoid arch the rudimentary acoustico-facialis. The latter is in contact with the anterior walls of the auditory sac. The rudimentary glosso-pharyngeal ganglion is noted above the third visceral arch. The vagus, or pneumogastric, ganglion is located above the fourth and the fifth. Note the form and the position of the heart. Note the anterior and the posterior limb rudiment in the trunk. In a study of the sections it is found that cuts transverse to the trunk pass about horizontally through the forebrain. At this age the following sections of the embryo should be studied: First, through the hindbrain, at which level will be noted the auditory sacs, the neuromeres, the trigeminal, the acoustico-facialis, and the glosso-pharyngeal and the vagus ganglion. Second, through the upper part of the pharynx, at which level will be observed the midbrain, the hindbrain, the visceral pouches, the nerves, and the blood-vessels. Third, through the choroid fissure of the optic cups. Note the parts of the eye, and, on the other side of the section, the heart. Fourth, a study of a section through the olfactory pits. Fifth, a study of a section through the pancreatic and the hepatic diverticula. Sixth, a study at the beginning of the allantois through the hind-gut. At this age it is of interest to study the systems of organs. Observe the manner in which the splanchno-pleure folds to form the walls of the intestine. Note the commencement of the mesentery. Note that the foremost part of the alimentary tract is formed from the stomodeal invagination of the ectoderm. The hypophysis is formed from a dorsal outgrowth of this. Note its relation to the brain. The following structures are formed from outgrowths of the ectoderm at this stage: First, the visceral pouches. Second, the median rudiment of the thyroid. This is an outgrowth from the pharynx between the two hyoid arches. Third, the rudimentary lungs, which develop in a pair from a median ventral diverticulum of the alimentary tract, just behind the last visceral pouch. The esophagus is just posterior to this. The esophagus, very short at this stage, is continuous with a slightly wider part that develops into the stomach.