THE CLOACA IN BIRDS 187 
ing to reconstitute themselves into a compact epithelial bud— 
a process which Jolly has compared to the production of Hassal’s 
corpuscles in the thymus. As involution continues the follicles 
separate from the epithelium and become replaced by fibrous 
tissue, the whole process taking place progressively from apex 
to base of the bursa in such a way that a gradual but rapid dimi- 
nution of volume and weight ensues. During the eighth month 
the bursa loses all possibility of functioning, and in the course 
of the next two months becomes reduced to a thin-walled cyst, 
still opening into the cloaca at its posterior end, but so completely 
fused to the aponeurosis of the rectum that it can be detected 
only by careful dissection. In this condition it may persist 
until old age. Only in the Ratitae, according to Forbes, does 
it remain as an undiminished organ throughout life where, by 
virtue of its broad opening into the proctodaeum, it becomes a 
repository for the urine. In these birds, according to Gadow, 
micturition and defecation are separate processes, whereas in 
most other birds the urine backs up into the coprodaeum and 
there mixes with the faeces until evacuated. 
The following table, arranged from data submitted by Jolly, 
is introduced to summarize the growth and involution of the 
bursa in the fowl: 
Ane Length Weight 
mm. grams 
5B Gi 3) LSU ae eae Ue ek Sailer Re A ga a 5 0.05 
Ilse Wael. sine can Sata Pek REET Ee io eeD ree ea ie 10 0.50 
2A WYNN Des SOS hae SSE ROLE See e eT TLE 15-18 0.50-1.0 
SHoT COVE AN ANSE ets 485 oe Spe ca oe as tL EG ae ee OR VT 20-25 1 6 
MERILOREUHS Meee ee ee ee ees eae ine § 30 3.0 (gn of body) 
A AN OIG Se Ee ENS eee ch ie Sea 2.51 
OEM On Hs ey 5. cieypaeeryseneitus sires Set titerons Tete tate ooh 0.97 
GET OMG S secs cee eS ts aie ote oto es eee rors 0.22 
Fy SEUCG TTI IE stag Utes A pt SR a iE 10-20 0.26 
Lem On hh ewes s wae GPA eokede fo eles fh ee ees 0.12 
The function of the bursa has never been satisfactorily ex- 
plained. Jolly’s description of the haematopoietic foci of the 
bursa, from which he derives not merely lymphocytes, but also 
red corpuscles and granular leucocytes, has added something to 
our knowledge of its activity, but, as he well recognized, this 
