THE DEVELOPMENT OF THE FIiOG\S EGG 45o 



rolled in to form the })cginnings of the food canal (Fig. 407 H). 

 Next, on the future dorsal side of the embryo, two ridges arise 

 with a furrow between them ; they are the beginnings of the 

 brain and spinal cord (Fig. 409). When the furrow is closed 

 by the ridges folding over it, the central nerve tube is formed 

 and begins to sink into the body. Later the trunk elongates, 

 nerves and muscles are formed, gills sprout out, and the tad- 

 pole is formed (Fig. 410). 



Fig. 409. — Two stages in the development of the frog while the brain and 

 spinal cord are arising./ From Morgan, "Development of the Frog's 

 Egg." 



Effect of Heat and Light on Development. — Eggs devel- 

 oping in normal environments in nature arrive at nearly the 

 same end-result, even when the environments are not identical. 

 We gain, in consequence, an impression that develojjment 

 proceeds unaffected by any changes in the outside world. 

 But this is not true. For one thing, the rate at which the devel- 

 opment of frogs' eggs proceeds depends closely upon the tem- 

 perature of the water. They develop most rapidly at about 

 30-32° C. If the temperature is elevated above this point, 

 the rate of development is retarded, and frnalh^ ceases at about 

 40° C. So, likewise, as the temperatiu-e is lowered, the devel- 

 opment is retarded, until at the temperature of freezing water 



