xii FERTILISATION 197 



the jelly surrounding the eggs when laid (p. 9). 

 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 ccelome, 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 a little sphere of jelly secreted by the 

 gland-cells. Passing down the oviducts the eggs accu- 

 mulate in the dilated extremities, which they distend 

 enormously, so that, just before laying, the abdomen of 

 a female frog is nearly filled by 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. 



Fertilisation. The eggs are now laid, and imme- 

 diately the spawn is passed from the oviducts of the 

 female into the water, the male sheds over them a quan- 

 tity of spermatic substance (p. 9). The sperms, swim- 

 ming actively through the water, enter the spheres of 

 jelly and come into contact with the eggs. A single 

 sperm then penetrates the egg-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. We 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 fertilisation or impregnation. 



