DEVELOPMENT OF THE FROG. 



127 



The neural folds first meet about the junction of the head and 

 trunk of the future tadpole, from which point the fusion extends 

 rapidly in both directions, forwards and backwards. The last 

 point at which fusion occurs is a little distance behind the 

 anterior end of the tube, at the place where the pineal body will 

 appear later. 



In front, the neural tube ends blindly : at its posterior end 

 it opens to the exterior at the blastopore, and is in free com- 



Fig. 30. 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-hranchial cartilage : 

 BF, fore-brain: BH, hind-brain : BM, mid-brain : C, coelom or body cavity : 

 C', pericardial cavity : CH, cerebral hemisj)here : CB, rudimentary cere- 

 bellum : CP, choroid plexus of fourth ventricle : CP', choroid plexus of 

 third ventricle : F, pharynx : G, stomach : H, lung : I, infundibulum : 

 J, 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. 



munication with the mesenteron. {Cf. Fig. 28.) The short 

 channel of communication between the neural tube and the 

 mesenteron, i.e.^ between the nervous system and the alimentary 



