202 THE FROG 



CHAP. 



cells of earlier stages, which cover the whole embryo 

 with the exception of the yolk-plug: these constitute 

 the ectoderm (Fig. 64, G, H, I, ect). 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 enteron 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 ectoderm 

 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, J, and 

 65, md. gr, md. /). The medullary groove extends back- 

 wards 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 ; compare Part II, Chapter 

 XIII), 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 diencephalon and to the 

 cerebral hemispheres, produced in front into the olfactory 

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

 cercbri ; and the hind-brain to the medulla oblongata 

 and cerebellum. It will be seen (Fig. 65) that the 

 medullary groove is lined by ectoderm and that there- 



