THE BKAiN 



119 



the anterior portion being vei-tical, and that of the posterior 

 portion horizontal. (Fig. 27.) The posterior portion, or hind- 

 brain, BH, is wide from side to side, and has moderately thick 

 sides and flooi', but a thin roof ; it is continuous behind with 

 the spinal cord. 



The anterior or vertical portion has walls of nearly uniform 

 thickness in all parts. It is divided by a slight constriction. 



Fig. 29. — Longitudinal vertical section through the head and 

 anterior part of the body of a tadpole about the time of appearance of 

 the hind legs. Length of tadpole, 12 mm. X 14. 



A, auricle of heart ; AD, dorsal aorta ; BB, basi-branchial cartilage ; 

 BF, fore-brain ; BH, hind-brain ; BM, mid-brain; C, ccelom or body 

 cavity; C', pericardial cavity; CH, cerebral hemisphere; CB, rudi- 

 mentary cerebellum ; CP, choroid plexus of fourth ventricle ; CP', 

 choroid plexus of third ventricle ; F, pharynx; G, stomach ; H, lung ; 

 1, infundibulum ; d, horny jaws; K, lip; L, liver; N, notochord ; 

 O, depression of floor of fore-brain from which the optic nerves arise ; 

 OE, oesophagus; P, pituitary body; PN, pineal body; S, central 

 canal of spinal cord ; T, truncus arteriosus ; V, ventricle. 



best marked at the sides, into an upper or posterior part, the 

 mid-brain, BM, which forms the angle of the bend and lies 

 opposite the anterior end of the notochord ; and a lower and 



