192 EDWARD A. BOYDEN 
SUMMARY 
This paper represents a review of the development of the 
cloaca in bird embryos from the third to the fifteenth day of 
incubation. It is based on the study of a large number of chick 
embryos supplemented by observations on three other species 
of birds. The most striking feature to be recorded is the regular 
occurrence of a temporary fenestra in the wall. of the cloaca, 
caused by the disintegration of a definitely localized area of 
epithelium and its subsequent removal by phagocytes, following 
which the contents of the cloaca are left in contact with the 
mesenchyma for a period of nearly twenty-four hours. It is 
of interest not merely because it furnishes the only instance in 
the differentiation of a hollow organ in which a gap occurs in 
the epithelial wall as a normal and constant feature of develop- 
ment, but also because it enables us, by virtue of the landmarks 
it establishes, to determine for the first time the exact point of 
origin of the bursa of Fabricius. 
The second part of this paper deals with the formation of a 
temporary sinus, placed athwart the main axis of the cloaca, 
which sinus has been interpreted as a repetition of the dorsal 
bladder of reptiles. This section also deals with some interesting 
anomalies growing out of the attachment of the wolffian ducts to 
the cloaca. 
A third feature of interest is the regular occurrence in chick 
embryos of an accessory bursal diverticulum (div. a), probably 
arising from the irregularities consequent upon the formation of 
the cloacal fenestra. By means of this diverticulum it has been 
possible to define the primordium of the bursa more accurately 
than has hitherto been done and therefore to offer new suggestions 
regarding its phylogenetic origin. 
LITERATURE CITED 
BornuaAvurpt, THropor 1867 Untersuchungen iiber die Entwickelung des 
Urogenitalsystems beim Hiihnchen. Riga. 
Boypren, Epwarp A. 1918 Vestigial gill-filaments in chick embryos with a note 
on similar structures in reptiles. Am. Jour. Anat., vol. 23, pp. 205-236. 
De Graar, R. 1668 De mulierum organis generationi inservientibus. Lugd. 
Batav., vol. 8, p.317. See Taf. 17, Fig. K. 
