DISUSE AND ITS EFFECTS. 4I 
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 
foetus 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 
