168 EDWARD A. BOYDEN 
now be defined as representing a persistence of the primitive 
streak in the form of a primitive-knot mass. From the cloaca 
to the tip of the tail it forms a deeply staining homogeneous mass 
differentiating above and below into epithelial structures and 
on the sides into the mesenchyma of the tail. The portion 
occupying the distal end of the tail is an active tissue giving rise 
to the medullary tube, caudal intestine, notochord, and other 
caudal tissues. The proximal half, on the other hand, is de- 
generating. Some of it may contribute to the mesenchyma of 
the tail, but most of it, as indicated by the presence of innumer- 
able phagocytes gorged with pycnotic nuclei, is undergoing re- 
sorption. This latter portion, representing an excess tissue, is 
absent from saurians and mammals, the caudal intestine in these 
forms lying close to the inner curvature of the tail. In this 
respect the cloaca of the tern (fig. 1) resembles that of lizards and 
snakes more than it does that of the gallinaceous birds. 
A second process which must be considered in relation to the 
formation of the cloacal fenestra is the disintegration of the caudal 
intestine. In all reptiles and mammals that I have examined 
and in one species of bird embryos (Sterna hirundo, the common 
tern) the caudal intestine undergoes reduction in the following 
manner. It appears to be pulled out, as if by the elongation of 
the tail, so that it tapers uniformly from the newly formed di- 
lated portion at the tip of the tail to a slender tube at the oldest 
portion—the region adjacent to the cloaca. As the latter por- 
3 The details of the process by means of which the primitive streak is segre- 
gated in the tail of the embryo will be described in a subsequent paper. At this 
time it is sufficient to state that the area described above is derived from that 
portion of the primitive streak which is included between the rhomboidal sinus 
and the anal plate of a fifteen-somite embryo. In consequence of the folding of 
the blastoderm, and of the accompanying overgrowth of the tail, the dorsal por- 
tion of the primitive streak, lying under the ectoderm, is folded into the outer 
curvature which forms the tip of the tail and thus becomes the tail-bud mass. 
The ventral half, lying above the entoderm, and therefore on the inner curvature 
of the fold, is tucked under the tail and compressed into the angle between the 
anal plate and the caudal intestine. 
4 This term of Koelliker’s seems more appropriate than ‘post-anal gut’ intro- 
duced by Balfour, since the gut-tract of the tail is an outgrowth of an area which 
originally lies anterior to the anal plate. As applied to mammals, the term is still 
less appropriate, as the caudal intestine disappears long before the anus is formed. 
