xn FERTILIZATION 197 



with gland-cells, which secrete the jelly (p. 9) surround- 

 ing the eggs when laid. Posteriorly it suddenly dilates into 

 a wide, thin-walled chamber (r. ovd") which opens into the 

 dorsal wall of the cloaca. Notice that there is thus no 

 connection between the generative organs and the kidneys 

 in the female, such as occurs in the male, the ureters serving 

 as renal ducts only. 



In the breeding season the ovisacs burst and set free the 

 eggs into the coelome, whence by some means or other they 

 find their way into the small openings of the oviducts, and so 

 into these tubes, when each becomes surrounded by its 

 little sphere of jelly secreted by the gland-cells. Passing 

 down the oviducts the eggs accumulate in the dilated 

 extremities, which they distend enormously, so that just 

 before laying, the abdomen of a female frog is nearly filled 

 with these two great egg-reservoirs ; the ovaries, having lost 

 so many of their eggs, are correspondingly reduced in size, 

 and all the other organs are squeezed out of place. 



Fertilization. — The eggs are now laid, and immediately 

 the spawn is passed from the oviducts of the female into the 

 water, the male sheds over them a quantity of spermatic fluid 

 (p. 9). The sperms, swimming actively through the water, 

 enter the spheres of jelly and come into contact with 

 the eggs. A single sperm then penetrates the vitelline 

 membrane of an ovum, loses its tail, and its head coming 

 into contact with the nucleus of the egg, fuses or conjugates 

 with it, so that a single nucleus is formed by the union of 

 the egg-nucleus with the sperm-nucleus. ^Ve may speak 

 of conjugating cells in general as gametes, the sperm in this 

 case being the male gamete, and the ovum the female 

 gamete, the body formed by the fusion of two gametes 

 being known as a zygote. 



This process is known as fertilization or impregnation. 



