118 



DEVEfiOPMENT OF THE FKOG 



present for a short time, and closes up before the tadpole 

 hatches. 



The neural tube, formed in this way, soon separates from the 

 surface epiblast, and by thickening of its walls and other 

 changes becomes converted into the central nervous system; 

 the anterior part forming the brain, and the posterior part the 



Fig, 28. — Longitudinal vertical section tiirougli tlie anterior end of 

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

 8 ram. 



A, auricle of heart ; BF, fore-brain ; 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 fore-brain 

 from which the optic vesicles arise ; OE, oesophagus ; P, pituitary 

 body ; PN, pineal body ; S, central canal of spinal cord : SO, stomo- 

 dseum ; T, truncus arteriosus ; V, ventricle ; Y, yolk-cells. 



spinal cord. The lumen or cavity of the tube persists as the 



central canal of the spinal cord and the ventricles of the brain. 



The Brain. At the time of its first appearance the brain is 



bent at right angles about the middle of its length ; the axis of 



