STRUCTURE OF THE VERTEBRATES 257 



is an enlargement formed at the original point of branching. 

 The pelvis narrows into the m^eter which passes backward to 

 its outlet from the animal's body. 



The major differences between the metanephros and the 

 mesonephros are: (1) the loss of the nephrostome; (2) the com- 

 plete loss of metamerism in its origin; (3) the loss of a meta- 

 meric blood supply and the development of paired renal arteries 

 which leave the dorsal aorta and reach the pelvis before divid- 

 ing; and (4) the development of new draining ducts and col- 

 lecting tubules. 



Urin.\ry Bladder. The urinary bladder is homologous with 

 the allantois. The latter develops as a ventral outpocketing from 

 the posterior gut, and its proximal portion is incorporated into 

 the cloaca. An allantoic bladder develops first in the amphibia; 

 but in the embryos of these animals the body wall is con- 

 tinuous (enclosing the yolk sac and allantois) and the allantois 

 is limited in size. In the adult amphibian this bladder is a 

 posterior pocket from the cloaca and the urinary products fall 

 into it as they pass from the mesonephric ducts. 



In the reptiles and birds the allantois grows out as the em- 

 bryonic breathing and excretory organ. AVhen the body stalk 

 closes after hatching, the proximal end of the allantois remains 

 as a distinct urinary bladder which has the same relative posi- 

 tion as in the amphibia. In the birds (with a few exceptions) 

 the bladder is merely a division of the cloaca, known as the 

 urodeum. The structure of the cloaca and bladder of the mono- 

 treme mammals is similar in all essentials. 



The marsupials and placentals develop an allantois which is 

 similar in development to that of the reptiles and birds; and, 

 as in the other amniotes, it is the proximal end of this out- 

 pocketing which develops into the urinary bladder. In both 

 groups during embryonic life the cloaca is present with its 

 allantoic pocket, the anus and urinogenital ducts opening into 

 it; but as development proceeds a constriction cuts the ventral 

 bladder from the more dorsal rectum. See diagram on page 255. 

 The ureters enter the bladder. The urethra which carries wastes 

 from the bladder to the outside is joined by the genital ducts 

 of the male. In the female the urinary and genital outlets are 

 separate in all vertebrates above the cyclostomes. 



