166 EDWARD A. BOYDEN 
In following the origin and fate of this particular foramen, to 
which I have applied the name cloacal fenestra, it became neces- 
sary to review the entire chain of events in thedevelopment of the 
cloaca from the formation of the primitive streak to the period of 
histological differentiation, and to supplement a quantitative 
study of chick embryos with observations on other species, 
notably duck, pheasant, gull, and tern embryos. As a result of 
this study a number of other interesting facts have come to 
light. Those relating to the early development of the hind-gut 
and tail have been reserved for a subsequent publication. 
DEVELOPMENT OF THE CLOACAL FENESTRA > 
In describing the origin of this foramen it will be necessary to 
refer occasionally to a peculiar tissue in the sacrocaudal region of 
young chick, pheasant, and duck embryos, which up to this time 
has not been observed in other birds or vertebrates. I refer to 
an indifferent cell-mass in the proximal end of the tail which 
persists long after the adjacent region has been differentiated—as 
late as the beginning of the fourth day of incubation in chick 
embryos. As seen in figure 5 (ps. v.), this inert mass lies within 
the angle formed by the cloaca and the caudal intestine, to both 
of which structures it is fused in a sagittal plane. Laterally it 
passes over into the mesenchyma of the tail, but rather abruptly, 
so that its limits can be approximately defined and the whole 
mass modeled in relation to surrounding structures, as displayed 
in figure 13. Beginning at the proximal end of the tail, this tis- 
sue is seen to be directly fused with the wall of the cloaca in the 
territory included between the anal plate and the junction of the 
caudal intestine with the cloaca (this being the wall of the cloaca 
which will later give rise to the bursa of Fabricius). Dorsally, 
this tissue is fused with the ventral border of the caudal intestine, 
and so intimately that the latter never has a chance to differen- 
tiate into an epithelium before it is resorbed. Ventrally, it 
fuses with the ectoderm bordering the anal sinus, while caudally 
it merges with the tail-bud mass—a fusion of three germ-layers 
extending across the tip of the tail. Thus the core of the tail 
is composed of an indifferent cell-mass, the whole of which can 
