THE CENTRAL NERVOUS SYSTEM. 



117 



of the epiblast, gives rise directly to the nervous elements, i.e. 

 to the nerve cells and nerve fibres, of the adult spinal cord. The 

 histological changes by which the nervous elements are formed 

 will be described in the chapters dealing with the chick and the 

 rabbit, in which animals they have been investigated more 

 completely than in the frog. 



The spinal cord extends to the extremity of the tail, which in 

 the later stages of tadpole life is of great length (Fig. 44, 9, 10, n). 

 During the absorption of the tail, at the time of the metamor- 

 phosis, fully two-thirds of the length of the spinal cord are lost. 



2. The Development of the Brain. 



The brain is merely the specialised anterior part of the 



BH 



FIG. 61. Sagittal section of a Frog Embryo, shortly after closure of the blas- 

 topore and formation of the anus, and of the same age as the embryo 

 shown in Fig. 58, E. x 25. 



BIT, fore-brain. BH, Hind-brain. BM, mid-brain. CH, notochord. M, meso- 

 blast. NC, cavity of neural tube. NT, neurenteric canal. PN, pineal body. PT, 

 ingrowth of epiblast which gives rise to the pituitary body. TI, intestinal region of 

 mesenteron. TP, pharyngeal region of mesenteron. U, proctodeeal or cloacal aperture. 

 W, liver, Y, yolk-cells. 



neural tube, and is directly continuous posteriorly with the 

 spinal cord. 



While the spinal cord is straight, or nearly so, the brain is 

 from its first appearance bent rather sharply, and nearly at 

 right angles, about the middle of its length ; the axis of the 

 posterior part being horizontal and continuous with that of the 

 spinal cord, and the axis of the anterior part vertical. The 



