THE CLOACA IN BIRDS 165 
garding the entodermal origin of the bursa. Since then only one 
paper has added any substantial increment to our knowledge of 
the general development of the avian cloaca, that of Pomayer 
(02), in the Fleischmann series, dealing especially with the 
development of the phallus. 
The present study originated with the discovery of a temporary 
foramen in the dorsal wall of the cloaca, produced by the disin- 
tegration of a definitely localized patch of epithelium and its 
subsequent removal by phagocytes, following which the con- 
tents of the cloaca are left in contact with the mesenchyma for 
a period of nearly twenty-four hours of incubation. This curious 
phenomenon was observed in over thirty embryos of the Harvard 
Collection, and its failure to occur has not been recorded in any 
embryos incubated approximately three days, the period at which 
the fenestra reaches its maximum size. My attention was 
first attracted to it by the presence of large numbers of embryonic 
phagocytes? similar to those found in the vestigial gill-filaments 
of chick embryos of corresponding age (Boyden, 718). Further 
study then demonstrated that this peculiar foramen was con- 
stant in its mode of development and invariably occurs, not only 
in chick embryos where it was first found, but in duck and pheas- 
ant embryos as well. It is of special interest not merely because 
it furnishes the only instance in the differentiation of a hollow 
organ, so far as I am aware, in which a gap occurs in the epithe- 
lial wall as a normal and constant feature of development, but 
also because it enables us, by virtue of the landmarks it estab- 
lishes, to determine for the first time the exact point of origin of 
the bursa of Fabricius. 
2 These cells were first described as degeneration cysts, but they were subse- 
quently seen in the underlying tissues into which they had been extruded from the 
epithelium, and were then recognized as embryonic phagocytes. It isa debated 
question whether these should be classed with the wandering cells of later embry- 
onic stages and thus derived from the mesenchyma in general (the macrophages, 
clasmatocytes, etc., of numerous authors), or should be considered to have arisen 
in situ as reactions of the local mesenchyma, or even of the epithelium itself, to 
the presence of dead protein. This problem will be discussed in another paper in 
connection with the appearance of phagocytes in the anal plate at so early a period 
as forty-eight hours of incubation. 
