258 



THE CHICK. 



blood-vessels penetrate, to form the choroid plexus of the third 

 ventricle (Fig. 116, XA). 



Immediately behind the stalk of the pineal body, the posterior 

 commissure is developed in the roof of the thalamencephalon, as 



RS 



BY 



BL 



XB 



V.I 



CH 



HR 



TN 



MC 



CH TO LR RC ES LT BB PT' HB 



FIG. 116. A median longitudinal, or sagittal, section of the head and anterior 

 part of the neck of a Chick Embryo at the end of the eighth day of incuba- 

 tion, x 10. 



BB, basibrancliial cartilage. BF, third ventricle, or cavity of the thalamencephalon. 

 BL, cerebellum. BM, Sylvian aqueduct, or cavity of the mid-brain. BS, lateral 

 ventricle, or cavity of the cerebral hemisphere. BT, lamina terminalis. BY, olfactory 

 lobe of the cerebral hemisphere. CH, notochord. ES, aperture of Eustachiaii tube. 

 ET, mesethmoid cartilage. FE, rudimentary feather. HB, basihyal cartilage. HR, 

 ceratohyal. IN, infuudibulum. K, epithelial knob on beak. LR, trachea. LT, 

 glottis. MC, Meckel's cartilage. NS, spinal cord. PN, pineal body. PT, pituitary 

 body. PT', stalk of pituitary body. RC, parachordal cartilage. TN, tongue. TO, 

 oesophagus. VI, neural arch of first or atlas vertebra. V2, centrum of second or axis 

 vertebra. XA, choroid plexus of third ventricle. XB, choroid plexus of fourth 

 ventricle. I, notch in mesethmoid cartilage for olfactory nerve. II, optic chiasma. 



a transverse band of nerve fibres connecting the two optic 

 thalami; it is shown, though not lettered, in Fig. 116. 



The floor of the thalamencephalon is depressed ventralwards 

 to form the infundibulum, which lies very close to the ante- 

 rior end of the notochord, and early acquires intimate relations 

 with the pituitary body. The infundibulum is already present 



