20 AN INTRODUCTION TO ZOOLOGY 



whitish ovoid body, the testis, will be seen on each side. It is 

 covered by a thin skin in which are a number of black pigment 

 cells. A bright yellow finger-shaped structure, the fat body, is 

 attached to the front end of each testis. 



The primary reproductive organs in the female consist of the 

 ovaries, which vary greatly at different times of the year. In the 

 early spring, before the egg laying has taken place, they are very 

 large indeed, and contain a large number of spheres, each composed 

 of black and white halves. These are the ova or eggs. After the 

 egg-laying period the ovaries are much smaller, but still of fair 

 size, and are long lobed structures of a greyish colour. A long 

 white coiled tube will be found dorso-laterally to the ovary, 

 extending from the anterior to the posterior end of the body cavity. 

 The oviduct, as this tube is named, conveys the eggs to the exterior, 

 and is much larger during the breeding season. 



Dorsal to the reproductive organs, and attached fairly closely 

 to the back of the coelom, are two irregularly oval bodies of a red 

 colour, the kidneys. These are concerned with the elimination of 

 the fluid waste material from the body. From the outer posterior 

 end of each a white tube, the ureter, passes backwards to open into 

 the dorsal wall of the cloaca, and this conveys the waste to the 

 exterior. In the male this tube swells out to form a sac, the 

 vesicula seminalis, in which the male reproductive elements are 

 stored until required. 



The whole of the pleuro-peritoneal cavity is lined by a 

 thin glistening membrane, the peritoneum, more or less well supplied 

 with pigment cells. In the mid-dorsal line the membrane on each 

 side passes ventrally and forms a series of folds, the mesenteries, 

 each composed of two layers, which hold the various viscera in their 

 places. The layer on the body wall is termed the parietal, and that 

 around the various organs the visceral layer of the peritoneum. 

 Where the two sheets of parietal peritoneum are reflected ventrally 

 to form the mesentery a space is left, the sub-vertebral lymph sinus. 

 In this are situated the main artery of the body, the dorsal aorta, 

 and a large vein that has its origin between the kidneys, the post-caval 

 vein. 



On the dorsal side of this sinus in the middle line will be found a 

 jointed bony rod, the backbone or vertebral column. 



If the frog now be turned on its back and the skin removed, 

 the vertebral column will be seen in the middle line almost com- 

 pletely hidden by muscles. These can be scraped off and the dorsal 

 part of the backbone carefully snipped away with a strong pair of 

 scissors. It will be found that the column possesses a distinct 

 cavity, the neural canal, in which is situated a soft rod-like structure, 



