114 ANIMAL BIOLOGY 



veins and so finally into the left auricle and thence into the 

 ventricle. Here it tends to become somewhat mixed with 

 the blood from the right auricle which has just returned from 

 the body. However, the structure of the heart and the ar- 

 teries is such that the blood that has come from the lungs with 

 a larger supply of oxygen is sent out by arteries to all parts 

 of the body. 



In the capillaries the oxygen is absorbed by the cells. 

 Oxidation of the food and protoplasm takes place and en- 

 ergy is thereby released, which enables the frog to carry on 

 locomotion, secure its food, and perform all its destined tasks. 

 The carbon dioxid and other wastes produced by oxidation 

 pass through the capillary walls into the blood and, as we 

 have seen, are carried back to the heart and then to the lungs, 

 where carbon dioxid is excreted. Other wastes are excreted 

 by the kidneys. 



The skin of the frog is likewise permeated by a network 

 of capillaries so that it acts as do the gills of fishes in ab- 

 sorbing oxygen from the water and in giving off carbon 

 dioxid. While the frog is buried in the mud during the winter 

 it breathes entirely through the skin. So much does the 

 frog depend on the skin as a breathing organ that even in 

 summer, if the skin becomes dry so that air cannot be ab- 

 sorbed, the frog dies. 



86. Reproduction and life history. — In the animals 

 studied thus far we have found special organs devoted to the 

 process of reproduction, namely, ovaries for egg production 

 in the female and in the male spermaries that form the 

 sperm-cells. Before the egg-cells can develop into embryos 

 each must be fertilized by a sperm-cell. All the facts we 

 have just stated apply equally well to the frog. 



Frogs' eggs are deposited in springtime in masses that 



