OUTLINE FOR LABORATORY STUDY OF THE CHICK 335 



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- oi 

 the two dorsal aortae, the appearance of the amniotic folds. Ob- 

 serve in the mesoderm the posterior cardiac veins, and the myo- 

 tomes, 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? 



Study sections through the auditory pit. Note fusion of the gill 

 pouches with the ectoderm. Note the blood-vessels. Study sec- 

 tions through the region of the heart, through the roots of the vitel- 

 line 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 SLXTY-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. De- 

 termine 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 diver ticulum, the 

 epiphysis. The mesencephalon, or midbrain, forms the apex of 



