THE CLOACA IN BIRDS 181 
of the ureter and the cloaca begins to develop irregular enlarge- 
ments sometimes suggesting diverticula (fig. 17), which eventually 
result in the widening of that segment. By the eighth day the 
distended ends of the wolffian ducts have been taken up in the 
urodaeal sinus as far as the origin of the ureters, the latter ducts 
in this process rotating from the dorsal to the mesial border of 
the wolffian duct. From this period on, the original components 
lose their identity in the sinus. In the adult the depth of this 
cavity is greatly reduced, the whole forming a shallow transverse 
segment, the definitive urodaeum, the latter being separated 
from the coprodaeum by the urorectal fold of Retterer and from 
the proctodaeum by the uro-anal fold. The position of these 
folds in the embryo is evident as early as the beginning of the 
fourth day of incubation. 
Another interesting feature of the urogenital apparatus which 
occurs at this time is the constriction of the metanephric pelvis 
at its lower third into a narrow isthmus (fig. 39). This was fig- 
ured by Schreiner (’02), who noted its relation to the umbilical 
arteries. Asis well known, the adult kidney of birdsisconstricted 
into three lobes. The cause of the upper constriction is yet to 
be determined; the lower constriction is accounted for by the 
mechanical obstruction offered by the umbilical arteries. The 
developing kidneys of the pig, as shown by Lewis and Papez, 
are similarly caught in the bifurcation of these vessels, but in- 
stead of becoming notched as in the bird, they escape by moving 
upward, sometimes, however, being brought so near together as 
to fuse from side to side, forming a ‘horseshoe kidney.’ 
In closing this chapter I wish to call attention to the changes 
which have been taking place in the terminal segment of the 
intestine. In figures 35 and 40 its lumen is shown to be occluded 
for some distance, the solid tube thus formed joining the uro- 
daeal sinus by a thin linear attachment. By the fifteenth day 
the cavity of the coprodaeum has been reéstablished and consid- 
erably distended except at the solid linear attachment. This 
greatly dilated chamber at the end of the intestine (fig. 41, 
copr.) is unquestionably homologous with the lower end 
of the rectum of the human foetus, as figured by Johnson (14). 
