310 MR. W. A. FORBES ON THE [Apr. 17, 
the following constituent parts :—(1) an external layer of connective 
tissue, covering (2) a thick layer of elastic membrane ; inside this 
is (3) a thin layer of mucous membrane, which unites together and 
connects the ‘‘ lymphatic follicles.” These “lymphatic follicles ” 
consist of masses of minute rounded cells, on an average 0°04 millim. 
Fig. 2. 

Back view of Cloaca of Uria troile. 
a. Bursa Fabricii. 0. Oviduct. c,¢. Ureters. d. Sphincter muscles. e. Czeea. 
in diam., enclosed in capsules of connective tissue, in which ramify 
their nutrient vessels &c. The lymphatic follicles are bound toge- 
ther by processes of the connective mucous membrane into raised 
processes, which project on the interior of the bursa, forming ridge- 
like “crests,” and are covered with epithelium internally, the cells 
of the latter being lanceolate with oval nuclei. In Rhea, however, 
the follicles are not closely bound together in masses forming ridges, 
but are attached by peduncles of elastic tissue to a central stem, the 
whole having somewhat the appearance of a bunch of grapes with 
a few berries on it. As we have already seen, Tannenberg in 1789 
was the first to point out that the bursa was more developed in young 
than old birds, it being gradually reduced and obliterated in the latter. 
This process of atrophy seems to obtain in all birds, so far as I can 
make out, though the periods of final disappearance seem to vary 
much in different groups. M. Martin St.-Ange found that the bursa 
began to lose its functional activity in Pigeons after six months, and 
in Fowls after eight; as a rule it seems to atrophy at about the 
period of full growth. On the other hand, in some cases it persists 
for long periods, and probably throughout life ; for I found it well 
developed and quite open in a specimen of Platycercus icterotis that 
