74 AN INTRODUCTION TO ZOOLOGY 



in it of a large number of spherical yolk granules, small masses serving 

 as food reserves for the subsequent development of the ovum. The 

 ovum is surrounded by a very thin homogeneous membrane, the 

 vitelline membrane, and held to the wall of the ovary by the epithe- 

 lium and connective tissue of the latter, which is reflected round it. 

 It thus comes to lie in a sac composed of epithelium covered by a thin 

 layer of connective tissue, and termed the follicle. In the course of 

 development the egg grows larger, and one half become impregnated 

 with a dense black pigment. 



The testes are compact bodies, and section shows them to 

 consist of a number of seminiferous tubules, whose walls are several 

 cells thick. The outermost layer contains ordinary cells, showing 

 little specialisation ; as they pass inwards, however, they become 

 more and more differentiated, as the result of a process known as 

 maturation, that will be considered in greater detail later. The inner- 

 most cells are very highly modified, consisting of a rod-shaped 

 head containing the nucleus, a short continuation of this, the middle 

 piece, and a long thread-like tail, capable of moving rapidly and 

 forming an organ by means of which the whole cell is able to 

 move about. These are the spermatozoa, or male reproductive cells, 

 actively motile cells with the power of existing and swimming 

 independently for a short time under suitable conditions. 



In the breeding season both male and female frogs' resort to 

 ponds and pools, and when the eggs are ready to be laid they associate 

 in pairs. The male takes up a position on the back of the female 

 and clasps her tightly by means of the pads on the first finger, which 

 have become enlarged and rough. As the eggs are laid the male 

 pours over them the milt or spermatic fluid, in which the sperma- 

 tozoa are contained. The sperm swims about actively until it 

 reaches an egg into which it penetrates, and with which it fuses. 

 Only one sperm can enter and fertilise an ovum, for immediately 

 after this has taken place the vitelline membrane undergoes a slight 

 alteration that prevents the entrance of other spermatozoa. Far 

 e more sperms are discharged than eggs, and consequently under 

 normal conditions practically all the eggs are fertilised. They are 

 now ready to develop into small free-swimming animals, tadpoles, 

 which after a period of growth turn into frogs. 



The kidney is composed of an enormous number of small 

 tubes, the uriniferous tubules, bound together by a small amount of 

 connective tissue richly supplied with blood vessels. Each tubule 

 commences as a Malpighian body, situated near the ventral side of 

 the kidney. This body consists of a hollow cup with double thin 

 walls, Bowman's capsule, the interior of which is filled up by a 

 glomerulus. The glomerulus is a rete mirable, formed by a branch 



