202 THE FROG CHAP. 



very definite manner. First of all there are several layers 

 of small pigmented cells, derived from the black cells of 

 earlier stages, which cover the whole embryo with the excep- 

 tion of the yolk-plug : these constitute the ectoderm (Fig. 64, 

 G, H, i, ecf). Forming the roof of the enteric cavity are 

 other layers of cells derived from the yolk-cells of earlier 

 stages and forming the endoderm (i, end} : the floor of the 

 archenteron is at present formed of unaltered yolk-cells. 

 Between the ectoderm and the yolk-cells on the lower 

 pole are several layers of small cells which gradually 

 spread until they form a complete layer between the ecto- 

 derm and the endoderm : these constitute the mesoderm (i, 

 mes). The ectoderm, endoderm, and mesoderm are known 

 as the three embryonic tissues or germinal layers. 



While these processes have been proceeding, a longi- 

 tudinal groove appears on the dorsal surface of the embryo : 

 this is the medullary or neural groove, and is bounded by 

 a pair of medullary folds (Figs. 64, H, and 65, md.gr, md.f). 

 The medullary groove extends backwards to the blastopore, 

 which now marks its posterior end. As development goes 

 on, the folds approach one another and unite, so as to 

 convert the groove into a canal : the cells lining this canal 

 give rise to the central nervous 'system (Fig. 64, K, and 

 compare Part II, Chapter XII), and its cavity becomes 

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

 brain. The anterior end of the hollow medullary cord thus 

 developed enlarges to form three vesicles, one behind the 

 other, and known respectively as the fore-brain^ mid-brain^ 

 and hind-brain^ each containing a ventricle. The fore-brain 

 eventually gives rise to the cerebral hemispheres, produced 

 in front into the olfactory lobes, and to the diencephalon ; 

 the mid-brain to the optic lobes and crura cerebri ; and the 

 hind-brain to the medulla oblongata and cerebellum. It 



