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 the pineal body the roof of the third ventricle, 



BM 



\ 



IN 



PT 



BY 



CB 



NS 



cr 



FlG. 151. A median longitudinal, or sagittal section through the head of a 

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

 view of an embryo at a slightly later stage.) x 10. 



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

 i)l>l(ingatii.or fourth ventricle 1 . BL, cerebellum. BM, cavity of mid-brain. BS, cavity 

 of cerebral hemisphere, or lateral ventricle. BT, lamina tenuinalis. BY, cavity of 

 olfactory lobe. CT, thyroid cartilage. ET, mesethmoid cartilage. F.I, first cervical 

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

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

 OB. Imccal cavity. QI, odontoid process of axis vertebra. OX, supraoccipital 

 cartilage. PF, jmsterior narial chaml)er. PL, palate. PN", pineal body. PT, 

 pituitary body. RP. basilar plate. TN, tongue. TO, oeso])hagus. TP, ptiarynx. 

 XA. choroid plexus of third ventricle. XB, ehoroid 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- 



