184 EDWARD A. BOYDEN 
where it is associated with the bursa of Fabricius (figs. 33 and 
34). The other diverticulum (6), when present, becomes 
associated with the urodaeal sinus (fig. 32, div. c). Both of them 
are probably to be regarded as irregularities produced at either 
end of the fenestra by the removal of intervening epithelium. 
They are present only in those birds which exhibit a fenestra, 
and are most conspicuous in that species which has the largest 
fenestra—the domestic fowl. The regularity with which di- 
verticulum a appears maty be explained by the fact that the 
posterior end of the fenestra is always larger, and that diverti- 
culum a, when first formed, arises from the prominence to which 
the primitive streak of earlier stages was attached (cf. figs. 21 
and 23). 
The later stages of development, which have been partly 
described by previous authors on the basis of sagittal sections, 
are shown in figures 34 and 39, 35 and 40, and 41. These models 
illustrate the development of the bursa up to the period of his- 
tological differentiation. The successive steps leading to this 
period are: 1) the continued outgrowth of the bursa and sim- 
ultaneous enlargement of its cavity through further coalescence 
of vacuoles; 2) the projection of the anal sinus (proctodaeum) 
in a ventrodorsal direction across the flanks of the urodaeum on 
its way to connect with the bursa (cf. figs. 18 and 39); 3) the 
breaking through of the thin plate separating the cavity of the 
bursa from the proctodaeum (ef. figs. 34 and 35), and, lastly 
(fig. 41), the differentiation into three parts of the passage-way 
thus made continuous from anus to the end of the bursa. At 
this stage (eleventh day) this passage-way is still separated from 
the rest of the cloaca by the urodaeal membrane, which does not 
rupture until after the seventeenth day (Gasser). As seen in 
figure 41, the first of its three parts, the proctodaeum of ecto- 
dermal origin, has assumed the shape of a compressed chamber 
with broad flange-like expansions. By the fifteenth day ecto- 
dermal glands have begun to differentiate around its circumfer- 
ence. The second and third parts, of entodermal origin, have 
developed, respectively, into a short bursal stalk and a greatly 
expanded but plicated sac, the bursa itself (fig. 41). The cavity 
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