Study sections through the auditory pit. Note fusion of the gill pouches with the ectoderm. Note the blood-vessels. Study sections through the region of the heart, through the roots of the vitelline veins, and through the primitive streak, if it is still present. For this study it will appear that the anterior end has developed in advance of the posterior end. The tail fold has probably just begun. Write a description of the pharynx, and of the circulation at this stage. 4. THE EMBRYO SIXTY-EIGHT TO SEVENTY-TWO HOURS OLD WITH CERVICAL FLEXURES FORMED. In a study of the living embryo note the changes visible to the naked eye since forty-eight hours old. Note the difference in the blood-vessels of the vascular area. Name the arteries and veins. Note the beating of the heart. In a study of the entire mount note that a second, the cervical flexure, has appeared in the head. Note that the tail fold is well formed. Note the position of the embryo on the blastoderm. Determine if the amnion is completely closed. Note the olfactory pits on the ventral surface of the head, a short distance in front of the optic stalks. Note the telencephalon, a rudiment of the cerebral hemispheres and an extension of the primary forebrain. It is bilobed anteriorly. The optic stalks are attached to the floor near the anterior end of the thalamencephalon. Note the infundibular region, which is the depressed region behind the optic stalks. In the roof of the thalamencephalon there is a short diverticulum, the epiphysis. The mesencephalon, or midbrain, forms the apex of the cranial flexure, and is united to the hindbrain by a narrow isthmus. The metencephalon, or rudimentary cerebellum, appears as a thick portion on the most anterior division of the hindbrain. The rest of the hindbrain is provided with a transparent roof and constitutes the myelencephalon, or the rudimentary medulla oblongata. Observe the inner and the outer wall of the optic cup, the lens, the choroid fissure, and the posterior, or vitreous, chamber.