THE CLOACA IN BIRDS ie 
a continuous plate (figs. 8 and 9, x) from a toc. In some cases 
the plate ruptures first through the distal diverticulum (see 
arrow in fig. 8); in others at first through the proximal one (fig. 
9). But in all chicks of older stages that I have examined, the 
plate is resorbed, leaving a single large opening from a toc. It 
is probable that phagocytes aid in this resorption, as I have found 
them within the thin plate as soon as the duct has joined the 
cloaca. As development proceeds, the lateral walls of the cloaca 
beginning with the anal plate gradually come together, forming 
a solid membrane comparable to the urethral plate of mammals, 
so that finally the opening of the wolffian duct becomes restricted 
to the middle of the cloaca at the level a of figure 6 (cf. figs. 14 and 
16). Not all of the complemental diverticula, however, fuse 
with the ducts. Some of them, no doubt, are soon suppressed. 
Others of them persist for a longer or shorter time, growing out 
as accessory diverticula (figs. 11, 22, 24, and 32, div.). 
The most interesting anomalies occur in duck embryos, and 
are due to the excessive length of the wolffian duct, which nor- 
mally grows down to the very end of the cloaca (fig. 7). In one 
case observed, only the proximal portion of the duct had fused 
with the cloaca, the terminal portion growing out as an aberrant 
diverticulum (fig. 10, ¢.p., left). In other cases both terminal 
and proximal portions fuse, but not continuously, so that an 
area of mesenchyma is left between the two attachments (fig. 7, 
mes.). If, then, the basal ends of the ducts begin to grow, 
a ring-shaped (fig. 10, ¢.p., right) or a U-shaped (fig. 11, t.p., 
left) attachment of the ducts is formed, opening into the cloaca 
at two points, representing the original points of fusion. A 
similar anomaly has been found in a chick embryo (H.E.C. 
99), and it would seem almost certain that a larger number of 
specimens would show many indications of aberrance resulting 
from the fusion of the wolffian duct to the complemental divertic- 
ula. The further changes in the form of the wolffian ducts and 
their incorporation into the wall of the cloaca will be considered 
in the next chapter. 
