286 BIOLOGY 



Fig. G, n. The result is the formation of a little animal such 

 as is shown in Figure H, in which the relation to the adult 

 structure can be clearly seen, although at this stage the em- 

 bryo only slightly resembles the adult frog. The development 

 that has taken place up to this point has occupied a period of 

 several days from the time when the egg was fertilized, the 

 exact length of time depending to a large extent upon the 

 temperature, the different stages being more rapidly passed 

 through if the eggs are kept warm than when they are kept cool. 



Various other systems of organs begin to appear at this stage 

 or a little later. From the ectoderm along the middle line in 

 the back, develops a rod of nervous matter, and around the 

 front end of this, outgrowths appear, which become the eyes, 

 ears, and other sense organs. The nervous mass itself differenti- 

 ates into the brain and spinal cord; Fig. H. The endodermal 

 tube also develops outgrowths which in time become the lungs, 

 liver, and pancreas. One part of the mesoderm forms itself 

 into a gelatinous rod running lengthwise in the back of the 

 embryo, just beneath the nervous system. This is the noto- 

 cord, nc; it represents the beginning of the spinal column, 

 and in time the vertebrce grow around it. Another part of the 

 mesoderm develops into the heart, ht, and blood vessels; while 

 that part of it which lines the body wall becomes the muscles, 

 and that which is next to the intestine develops into the peri- 

 toneum and mesentery. From the mesoderm, too, the kidneys 

 and sexual glands arise, with their ducts, s. 



These changes take place quite rapidly, although they are 

 not completed for many days. When they are finished the 

 whole series of the organs of the frog is present, though yet 

 incompletely developed. Meantime the animal has hatched 

 from the egg, and forces its way out of the jelly in which it 

 has been- embedded and assumes an independent life. 



5. Metamorphosis. The further development of the frog 

 comprises a number of different stages, shown in Figure 133, 

 the important features of which are as follows: The animal 



