MORPiroLoav of higher types 



127 



tubules (called uriniferous tubules) beginning in globular swellings, the 

 Malpighian corpuscles (Fig. 93). The Malpighian corpuscle is composed 

 of two parts, the glomerulus and Bowman's capsule. The glomerulus 

 consists of a group of blood capillaries. Surrounding the group of capil- 

 laries is Bowman's capsule which is the expanded and invaginated end 

 of one of the uriniferous tubules forming a double-walled cup. Each 

 tubule consists of a single layer of secreting epithelium lying throughout 

 its course in intimate contact with a rich supply of capillaries from the 

 circulatory system. In some of the lower vertebrates, as the lamprey 

 and the frog, there are ciliated funnels or nephrostomes (Fig. 94) situ- 

 ated on the ventral surface of the kidney and opening into the body cavity. 

 In the larvae of these species the nephrostomes connect the body cavity 

 with the uriniferous tubules but in adult frogs the nephrostomes are con- 



FiG. 94. — Diagrammatic representation of a cross-section of the kidney of a frog. B, 

 Bidder's canal. C, collecting tubule; Z), dorsal ;L, lateral margin of kidney; M, Malpighian 

 corpuscles; A'^, nephrostome; T, uriniferous tubule; U, ureter; V, renal portal vein. {Modi- 

 fied from Holmes's Biology of the Frog.) 



nected with the blood vessels instead of with the tubules. A diagram 

 of the structures found in the kidney of the frog is shown in Fig. 94. 



The kidneys discharge into ureters which in turn discharge into the 

 cloaca in lower vertebrates, as the frog, or into the urinary bladder in 

 mammals. In the frog the urinary bladder is located on the ventral side 

 of the cloaca but the ureters are not directly connected with it. Urine 

 discharged into the cloaca by the ureters collects in the bladder and 

 thence is discharged at times through the cloaca. The urinogenital 

 apparatus of a male frog is shown diagrammatically in Figs. 84 and 98. 

 In mammals, except the few which have a cloaca, the urinary bladder 

 is connected with the exterior by means of the urethra. 



Sexual or Genital System. — The sexual or genital system has for 

 its primary function the production of the germ cells. The transporta- 

 tion of the germ cells, their storage before and after fertilization, and 

 the protection or nutrition of embryos developing from the germ cells 



