THE BRAIN. 



375 



irregular hollow outgrowths arise : at a later stage these out- 

 growths become solid, and separate completely from the stalk. 

 In front of tlic pineal body the roof of the third ventricle, 



BV 



31 



PT 



MC 



OB 



Cl 



FK;. 151. -A median longitudinal, or sagittal section through the head of a 

 Kabbit Embryo of the eighteenth day. (Compare Fig. 149 for a surface 

 vic\v of an embryo at a slightly later stage.) x 10. 



BF, cavity of thalamencephalon, or third ventricle. BH, cavity of medulla 

 oblongata, or fourth ventricle. BL cerebellum. BM, cavity of mid-braiii. BS, cavity 

 (if cerebral hemisphere, or Intend ventricle. BT, laniiiiii terminalis. BY, cavity of 

 i.l factory lobe. CT, thyroid cartila.w. ET, mesethmoid cartilage. F-l, first cervical 

 or atlas vertebra. F.2. >eeond cervical or axis vertebra. IN, infundibulum. LR, 

 trachea. LT. glottis. MC. Meckel's cartilage. N"S, central canal of spinal cord. 

 OB, buccal cavity. OI, odontoid pi-ocess of axis vertebra. OX, supraoccipital 

 cartila-c. PF, posterior inirial chamber. PL, palate. PN, pineal body. PT, 

 pituitary body. RP. basilar plate. TN, tongue. TO, lEsophau'iis. TP, pharynx. 

 XA. choroid pteXTW of third ventricle. XB. choroid plexus of fourth ventricle. 



which is here excessively thin, becomes thrown into folds, which 

 hang down into the cavity of the ventricle (Fig. 151, XA). Be- 

 tween these folds blood-vessels pass in, from the vascular meso- 



