126 



DEVELOPMENT OF THE FROG. 



narrows posteriorly towards the blastopore. Slightly raised 

 ridges, the neural folds, soon appear, bordering the sides of the 

 neural plate ; and a longitudinal neural groove is formed along 

 its dorsal surface in the median line, extending forwards from 

 the blastopore. 



Fig. 29. Longitudinal vertical section through the anterior end of 

 a tadpole shortly after the time of hatching. Length of the tadpole 

 8imm. 



A, auricle of heart: BF, fore-hrain : BH, hind-brain: BM, mid-brain: 

 C, pericardial cavity : CV, vesicle of cerebral hemispheres : I, infundi- 

 bulum : L, liver : N, notochord : O, depression of floor of forebrain from 

 which the optic vesicles arise : OE, oesophagus : P, pituitary body : 

 PN, pineal body : S, central canal of spinal cord : SD, stomodaium : 

 T, truncus arteriosus : V, ventricle. 



A transverse fold connects the anterior ends of the neural 

 folds together, slightly raising up the anterior end of the neural 

 plate. The neural folds now grow rapidly : the groove between 

 them deepens, and the folds becoming more and more prominent 

 bend in towards each other (Fig. 27) and finally meet and fuse, 

 thereby converting the neural groove into a tube. 



