RESPIRATORY SYSTEM 771 



outlet. The organ used for this combined respiratory and excretory 

 function is called the allantois (Fig. 363). This is the ventral diverticu- 

 lum from the more caudal part of the digestive canal, which has already 

 been studied in Embryology. It becomes larger with the growing em- 

 bryo. It is extremely vascular and is absorbed in some forms such as 

 the Sauropsida or is drawn off with the placenta in mammals. The basal 

 part, however, persists as the urinary bladder. 



