THE ALIMENTARY CANAL 



557 



From the hindmost region of the gut, the allantois grows 

 out in all animals from Amphibians onwards. In Amphi- 

 bians it is represented by a cloacal bladder ; in the higher 

 Vertebrates it is a vascular foetal membrane concerned 

 with the respiration or nutrition of the embryo, or both. 



Fig. 316. — Section through a young newt. 



c.t., Connective tissue ; E., epidermis ; D., dermis ; S.C., spinal cord ; M., 

 muscle ; N., notochord ; Sh., mesodermic sheath of notochord ; K., 

 kidney ; I., lung ; S., spleen ; ST., stomach ; Pe., peritoneum ; L., 

 liver ; d., duct of the pancreas (P.) ; G.B., gall-bladder ; V., dorsal 

 aorta. 



CiUa are very common on the lining of the intestine in 

 Invertebrates, but they are much rarer in Vertebrates. Yet 

 as they occur in Amphioxiis, lampreys, many fishes, Proto- 

 pterus, some Amphibians, and in embryonic Mammals, it 

 seems not unlikely that the alimentary tract was originally a 

 ciliated tube. 



