186 EDWARD A. BOYDEN 
of the proventriculus (glandular stomach of birds) to which the 
bursa was compared in 1829 by Berthold. In the region next 
to the stalk, according to Schumacher (’03), the follicles are 
neither so thick nor so sharply limited, but look more like a 
diffuse infiltration of tunica propria with lymphocytes. To these 
finger-like processes, which in my model of the fourteen-day 
chick are restricted to the dorsal wall of the bursa where it joins 
the stalk, Schumacher has applied the term mucosal villi. 
The nature of the epithelial transformation has received several 
interpretations. Wenckebach (’88) and Schumacher (’03) main- 
tain that the entodermal epithelium constituting the medulla of 
each follicle is differentiated directly into lymphoid tissue, and 
that this process is followed by a differentiation of the mesenchy- 
mal cortex into a similar tissue, the border-line between the two 
layers becoming ill-defined in later stages. Retterer, in his 
latest paper (713), extends the activity of the epithelium still 
further, stating that ‘‘the cortex of the follicles of the bursa is 
likewise of epithelial origin.”” The most comprehensive account, 
however, is that of Jolly (15), who based his conclusions not 
merely upon histogenesis, but also upon the involution of the 
organ (both natural and induced) and upon examination of 
tissues in vitro. Beginning with the eleventh day of incubation, 
he finds numerous amoeboid cells, formed directly from the 
mesenchymal network, accumulating in the vicinity of the epithe- 
lial buds. These they soon invade, the most active phase of 
penetration occurring between the fourteenth and eighteenth 
days. Although at first the epithelial cells give way to the new 
arrivals, by becoming detached from one another and in some 
cases by even degenerating, the majority of them, he maintains, 
enter upon a symbiotic relation with the invaders by means of 
which both cell strains continue to divide actively, the amoeboid 
cells giving rise to large numbers of small lymphocytes, the 
epithelial cells forming a reticular network within which the 
lymphocytes reside. Simultaneously the cortex becomes differ- 
entiated into a highly vascularized lymphoid tissue. 
In involution the order of events is reversed; the lymphocytes 
in the medulla die and the epithelial cells close their ranks, tend- 
