174 EDWARD A. BOYDEN 
the fourth day of incubation all signs of the cloacal fenestra 
have disappeared, and its site cannot be accurately located ex- 
cept in such general terms as lying between the accessory bursa 
and the urodaeal sinus. 
In concluding this chapter one may say that the most conspicu- 
ous feature of the entire process is the rapidity with which it 
takes place—both the sudden appearance of a gap and the rapid 
closure of it—all occurring within a period of twenty-four hours. 
Although the evidence presented would lead one to infer that 
the disintegration of the cloacal wall precedes the reduction of 
the caudal intestine, and is thereby independent of it, and calls 
for a separate explanation, it is still possible that the cloacal 
fenestra represents a modification or extension of the process by 
which the caudal intestine is reduced in other vertebrates. Any 
attempt, however, to explain the significance of this foramen in” 
the domestic fowl, duck, and pheasant, must take into account. 
an equally peculiar feature, likewise found only in birds with a 
fenestra, namely, the undue persistence of the primitive streak 
in the proximal end of the tail. It is well known that the tail in 
modern birds, and of fowls in particular, is shorter than in the 
‘Archaeornithes. It is conceivable that the degenerating primi- 
tive-streak mass in the tail of the chick embryo represents a 
persistence of material once utilized in tail-building but now 
superfluous. It would also seem, from a comparison of the clo- 
acas in the first text plate, that the persistence of this indiffer- 
ent tissue hasdelayed the differentiation of the caudal intestine and 
perhaps of the whole tail itself. For figure 5 represents a chick 
embryo in which the ventral wall of the caudal intestine has 
not been differentiated into an epithelium, but is still continuous 
-with the primitive streak throughout its length. Yet that chick 
is older in other respects than the tern embryo of ‘figure 1, as 
evidenced by the lesser number of somites in the chick, and by 
its greater maturity of form. If it be granted that the devel- 
opment of the caudal intestine in the chick has been retarded by 
the persistence of the primitive-streak mass, it is not inconceiv- 
Y 
é 
