DISUSE AND ITS EFFECTS. 41 



organ of the sauropsidian embryo. In the embryo of 

 placental mammals the work of respiration is considerably 

 modified, being performed by means of the placenta, an 

 organ formed of structures derived in part from the 

 fetus and in part from the mother. Under these 

 conditions the function of the allantois is limited to 

 the conveyance of blood-vessels from the embryo in 

 order to bring them into intimate relation with the 

 maternal tissues. This work accomplished, the allantois 

 shrivels, with the exception of the part in relation with 

 the cloaca ; this becomes permanently useful in mammals 

 as the urinary bladder ; a portion, however, remains as a 

 withered cord passing from the summit of the bladder to 

 the navel and is known as the urachus. 



Thus the allantois exhibits what at first sight appears 

 to be a change from a respiratory organ to a receptacle 

 of urine, but closer inquiry shows the matter to be some- 

 what different. Amphibians possess a urinary bladder, 

 but not an allantois : a critical inquiry into the matter 

 induces me to accept Balfour's view and to look upon 

 the allantois as an enormously enlarged urinary bladder 

 which assumed in the embryo respiratory functions. 

 This change is coincident with, if not responsible for, 

 some extraordinary alterations. Fish and amphibia 

 (Ichthyopsida) differ from reptiles and birds (Saurop- 

 sida) and mammals in that they possess during some 

 period of their lives, gills, and in the non-possession of a 

 functional allantois. No vertebrate is known which 

 possesses gills and a functional allantois. Before the 

 advent of the allantois, embryonic respiration is carried 

 on in a variety of ways, sometimes by external gills, as in 



