The dorsal and the ventral roots of the spinal nerves are given off separately, and secondarily unite. From the neuroblasts of the cord there are at regular intervals outgrowths representing the ventral roots. From the neural crest there develop segmental collections of neuroblasts which form the spinal ganglia, from which the dorsal spinal nerve roots develop. In fact this developmental stage can be observed in embryos only forty-eight hours old, first appearing as a line of cells springing on each side from the angle between the neural canal tube and the external epiblast. In the section from the embryo seventy-two hours old there are observed the rudiments of the development of these spinal nerves. Four primary ganglia develop in the neural crest of the head. These ganglia are as follows: the acoustico-facialis ganglia, which is located over the hyoid arch; the ganglia of the trigeminus, which is located over the mandibular arch; the ganglia of the glosso-pharyngeus, which is located over the third visceral arch; and the ganglia of the vagus, which is located over the third and the fourth visceral cleft. In this stage of development the trigeminal and the acoustico-facialis are clearly visible. On each side and dorsal to the aorta is noted the Wolffian body, or mesonephros. The Wolffian body (Fig. 82, _E_) consists of a series of tubules imbedded in the mesenchyme. The openings into the Wolffian duct lie just beneath the cardinal vein. In a study of the =Wolffian duct= determine just how far anteriorly and how far posteriorly it extends. Note whether it empties into the cloaca. Each tube beginning in a blind extremity is later dilated. It has a thin wall, and is situated near the median portion of the Wolffian body. The tubule proper passing transversely, opens into the duct. The upper wall of the thin walled part is invaginated by a mass of mesenchyme that receives a small vessel from the dorsal aorta. The Malpighian corpuscle, consisting of a glomerule and Bowman’s capsule, is thus established. In the study of the four days old chick note the further development of these parts. In a study of a chick four days, or ninety-six hours old, note to what extent the yolk is covered by the blastoderm. It will be noted that the embryo lies in the extra-embryonic cavity. This cavity is bounded above by the chorion and below by the splanchno-pleure. In removing the amnion from the embryo, note the relation to the somatic umbilicus. Note the relation of the splanchnic umbilicus to the splanchno-pleure. What relation has the allantois to the above? In examining the head, locate the cerebral hemispheres, and note their development. Locate the pineal gland. Note changes in the olfactory pit and the eye. Locate the lens and the choroid fissure. Note that the maxillary process of the mandibular arch lies beneath the eye and behind the olfactory pits. Note the otocyst and the relations of the other arches to the above structures.