810 COMPARATIVE ANATOMY 



THE METANEPHROS. 



While the mesonephros functions in all vertebrate embryos and 

 throughout the entire life of fish and amphibians as well as a short time 

 after birth in the lizards and opposum, this organ becomes replaced in 

 the adult of all amniotes by the two metanephridic organs which form 

 the true kidneys. Each of this pair of kidneys takes its origin directly 

 behind the mesonephros of the same side, while from the caudal end of 

 the Wolffian duct close to its entrance into the cloaca a tube, the ureter, 

 grows forward parallel to the parent duct, into the tissue caudal and 

 dorsal to the mesonephros. It is supposed that this is more or less 

 metameric, although all trace of such metamerism has disappeared, the 

 kidneys not being segmented in any stage of their development. The 

 cephalic end of the ureter has a varying number of branches, whose tips 

 expand to form what is called a primary renal vesicle. Around each 

 primary vesicle a group of cells develops, the aggregate of which grows 

 into an "S" shaped tubule, one end of which connects with the primary 

 renal vesicle, the other developing into a glomerulus. There are no neph- 

 rostomes. Still later these tubules multiply extensively, while the blood 

 capillary system of the glomerulus increases also. 



THE URINARY BLADDER. 



This reservoir for urine, also called a urocyst, forms toward the 

 caudal end of the excretory ducts. There are three kinds of urocysts. 



1. A bladder arising by the fusion of the caudal ends of the Wolfrian 

 ducts plus a portion of the digestive tract. This is the cloaca type. The 

 Wolfrian ducts in this instance empty into the cloaca, the cloaca then 

 opening to the exterior. 



2. The usual urinary bladder formed by a diverticulum from the 

 dorsal wall of the cloaca, cephalad to the openings of the Wolfrian ducts. 

 It is supposed however that this form may be homologous with the 

 rectal gland of the elasmobranchs. 



3. The allantoic bladder occurring in all higher forms as a ventral 

 diverticulum from the cloaca. The entire outgrowth forms the bladder 

 in the amphibia, while in the amniotes only the proximal portion becomes 

 the bladder. The distal portion is used in the embryo as a respiratory 

 organ, the allantois. The allantois is quite extensive, forming a part 

 of the placenta in mammals. It is either absorbed or lost at or before 

 the time of birth. 



In the higher forms in which a bladder is present, the ureters open 

 directly into it, the urine being conveyed to the exterior through the 

 single tube, the urethra, while in amphibia the urine must first pass 

 through the cloaca before entering the bladder, as the Wolffian ducts 

 do not directly enter the urocyst. In many birds and reptiles there is no 

 urinary bladder at all although these have an allantois during their em- 

 bryonic development. 



"There is great difficulty in comparing the excretory system of the 

 vertebrates with anything known in the invertebrates. In general the 



