The Development of the Frog 35 



Of the changes that convert the simple tube 

 into the adult cord little need be said. The 

 walls thicken rapidly, and the originally large, 

 circular canal is reduced to a narrow, vertical 

 slit (Fig. 15, NS). As development proceeds, 



FIG. 13. TRANSVERSE SECTION THROUGH A FROG 

 EMBRYO DURING THE FORMATION OF THE NEURAL 

 CANAL. (After Marshall.) 



C, ccelom. E F^ epidermic layer of ectoblast. E N, nervous 

 layer of ectoblast. //, entoblast. M, mesoblast. M E, somato- 

 pleuric layer of mesoblast. M //, splanchnopleuric layer of meso- 

 blast. M ' N, mesenteron. jV, notochord. N F, neural fold. 

 JVC, neural groove. Y, yolk-cells. 



the relative size of the canal becomes smaller 

 and smaller until, in the adult, it is seen as 

 a tiny tube in the centre of the cord, lined 

 with a thin layer of epithelial cells, derived 



