170 EDWARD A. BOYDEN 
The first indication of impending disintegration appears in a 
duck embryo of forty-five somites (fig. 2). Two paired foci of 
degeneration (y and z) are here disclosed in the cloacal wall, one 
near the junction of the caudal intestine and cloaca, the other 
just anterior to the orifice of the wolffian duct. It is probable 
that area y is the first to develop as it is present on both sides of 
the cloaca, while area z is present only on the left side. This 
specimen, if corroborated by more examples, would seem to 
indicate that the degenerative process, which later involves the 
caudal intestine, begins in the wall of the cloaca near its junction 
with that structure. . 
In the next stage (fig. 4, of a duck embryo two somites older), 
the two areas on each side have grown together, presenting a 
continuous line of degeneration. In addition the lumen of the 
caudal intestine has become occluded (fig. 4, x), in the region 
which corresponds to the point of rupture in other vertebrates. 
This observation is important as indicating the independent ori- 
gin of the two processes—the resorption of the caudal intestine 
and the formation of a cloacal fenestra—and shows that in the 
duck, at least, the caudal intestine becomes detached slightly in 
advance of the production of the fenestra. In this specimen, 
what remains of the undifferentiated primitive streak (ps. v.) 
is appended to the caudal intestine. In the embryo shown in 
figure 2, which is younger in other respects, all the primitive 
streak has been removed, its former presence being indicated 
only by the roughened and irregular ventral margin of the caudal 
intestine. 
The third stage, illustrated by a chick embryo of forty-one 
somites (fig. 5), shows an extension of the area of degeneration 
both caudal and cephalad,® and the appearance within this area 
5 The cephalic extension contains only scattered phagocytes (represented by 
periods in the figure) and does not usually become denuded of epithelium, 
although the fenestra has been observed to extend that far in a few cases. If the 
cut end of the rectum in figure 5 be examined, it will be noticed that the periods 
are limited to a zone of the cloacal wall which is thinner than the adjacent zones. 
This area, together with the dorsal wall of the caudal intestine with which it is 
continu us and homonymous, represents a persistence of the primitive condition 
of the hind-gut which, like the roof of the foregut, is always thin-walled when first 
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