412 ARTHUR ROBINSON. 
broad curved plate (fig. 20, Pl. XX VII,), which lies between 
the paraxial plates of mesoblast, and is directly continuous 
with the remainder of the hypoblast; this is succeeded by a 
thickened rod of hypoblast (fig. 20 A), which bears the same 
relation to the mesoblast and the remainder of the hypoblast 
as the curved plate in front of it. The rod-like mass is 40 
long. It terminates posteriorly in the anterior end of the 
primitive streak, where it is fused dorsally with the epiblast, 
and laterally with the mesoblast (fig. 20 B). 
At the eleventh day, when the neural groove is closed from 
the lumbar region to the mid-brain, and the folding of the 
posterior end of the embryonic area has resulted in the 
enclosure of the posterior part of the gut, the hypoblastic 
ridge is entirely separated from the remainder of the hypo- 
blast, except at the dorsal end of the bucco-pharyngeal mem- 
brane in front, and at the anterior extremity of the primitive 
streak behind (figs. 21 and 21 A). In the latter situation the 
posterior end of the ridge is not only continuous with the 
dorsal wall of the gut (figs. 21 and 21 A), but also with the 
mesoblast of the primitive streak, and through it with the 
epiblast ; the connection is more evident in transverse than in 
longitudinal sections (fig. 21 B). 
The folding of the posterior end of the embryonic area 
mainly affects the region of the primitive streak, which is bent 
upon itself twice; first at the junction of its anterior and 
middle thirds, and secondly at the junction of its middle and 
posterior thirds. The upper surface of the anterior third 
enters into the formation of the neural canal, and after the 
fusion of the medullary folds it is entirely shut off from the 
surface of the body. 
The superficial surface of the middle third of the primitive 
streak remains on the surface of the body between the base of 
the tail and the anus. 
The anus is formed through the upper part of the posterior 
third of the streak, where at the eleventh day the anal 
membrane is very clearly defined (fig. 21). 
I have not yet been able to trace the early stages of the 
