446 ARTHUR ROBINSON. 
rapidly backwards from the posterior end of the primitive 
streak, along the under surface of the somatopleure. In 
respect to its position, the allantoic mesoblast of the human 
ovum does not differ from the allantoic mesobiast of other ani- 
mals, for in all cases this mesoblast is attached to the somato- 
pleure, which becomes the ventral wall of the embryo from 
the umbilical orifice to the anterior wall of the rectum. The 
precocity of its appearance, before the formation of the caudal 
amnion fold, prevents the necessity of its growth as a free 
stalk through the extra-embryonic ccelom, in order that it 
may attain a connection with the trophoblast, for as soon as it 
passes the posterior limit of the epiblast it becomes at once 
attached to the trophoblast, therefore its extra-embryonic por- 
tion is comparatively extremely short in the early stages of 
the human ovum, and after the completion of the ccelom the 
embryo does not lie in the centre of the ovum, but close to 
that portion of the wall to which it is attached (21, fig. 9 a, 
p. 145). 
The rapid growth of the allantoic mesoblast and its early 
attachment to the trophoblast interfere with the formation of 
the ceutral portion of the tail amnion fold by preventing the 
dorsal extension of the body-cavity at the posterior end of the 
embryonic area. The forward extension of the cavity is not 
affected, and as soon as its lateral horns have fused in front of 
the embryonic area it rapidly increases both ventrally and 
dorsally. The dorsal extension is most rapid at the anterior 
end, consequently the cephalic amnion fold rises, and is carried 
backwards over the embryonic area. The lateral folds and the 
lateral portions of the caudal fold rise more slowly, and the 
closure of the amnion navel takes place at the posterior part 
of the embryonic area, in a situation which afterwards be- 
comes the posterior boundary of the umbilicus. When 
this closure is completed the epiblast is separated from the 
trophoblast (His, 15, Taf. ix, figs. 1 and 3; Taf. xii, fig. 4), and 
the dorsal surface of the “‘ Bauchstiel” forms part of the boun- 
dary of the amniotic sac—that portion which extends, when 
the curvature of the embryo is completed and the umbilical 
