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, 

 (1, H, I, eci). Forming the roof of the enteric cavity are 

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

 stages and forming the oidnderm (I, end) : the floor of the 

 archenteron is at present formed of unaltered yolk-cells. 

 Between the ectoderm and the yolk- cells on the ventral 

 aspect 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. 



\\'hile these processes have been proceeding, a longi- 

 tudinal groove appears on the 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 marks the dorsal surface of the 

 embryo and extends backwards to the blastopore, which 

 now marks its posterior end. i\s 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, f.br, m.hr., h.hr, 

 sp.cd), and its cavity becomes the central canal of the spinal 

 cord and the ventricles of the brain. The anterior end of 

 the hollow mcdullarv cord thus developed enlarges to form 

 three vesicles, one behind the other, and known respectively 

 as \h& fore-brain, niidd?rain, and hindd'rain, each containing 

 a \entricle. The fore-brain e\entuall)' gives rise to the 

 (erebral hemispheres, produced in front into the olfactory 

 lobes, and to the diencephalon ; tlie mid-brain to the optic 

 lobes and crura cerebri ; and the hind-brain to the medulla 



