476 Description of a 4-mm. Human Embryo 
The shape in cross section can be seen in Fig. 8; the opening of the canal 
to the outside is artificial, a tear in the section. The spinal cord follows 
the curve of the rump, passing nearly out of sight in the undissected 
part, but reappearing nearer the tail. Here a change has taken place. 
The ectoderm of the spinal cord and that of the skin have been gradually 
becoming continuous, and just after rounding the curve of the rump the 
medullary tube is replaced by an open medullary groove. This opening 
of the spinal cord to the outside is accompanied, as in the case of the 
anterior neuropore, by a rolling of the edges, as shown in Fig. 16, and in 
section in Fig. 8. The groove becomes wider, until the sides lie in the 
same plane, but the medullary plate is distinguishable from the ectoderm 
of the skin, almost to the end of the tail, by its thickness. At the tip of 
the tail, which has been cut off in the model, all the tissues become 
merged into an undifferentiated mass. 
Notochord.—The notochord, shown in the back region in Fig. 15, and 
in the tail region in Fig. 16, lies just ventral to the spinal cord to which 
at times it seems attached (Fig. 8). In cross section it is at first rounded, 
but at the curve of the rump becomes oval, with long axis dorso-ventral, 
and considerably larger. It follows the curve of the spinal cord, and is 
lost not far from the end of the tail by merging with the spinal cord 
(Fig. 16). 
Aorta—tThe aorta lies ventral to the notochord, separated from it by 
a narrow strip of mesenchyma. Att first, i. e., in the back region, it sends 
off small buds, not always directed laterally, apparently not segmentally 
arranged, but presumably representing intersegmental arteries. At the 
apex of the rump curve the aorta, now entirely separated from its fellow 
on the opposite side of the body, leaves the notochord and curves sharply 
forward. At this point a branch is found, extending laterally and toward 
the spinal cord, mesial to the Wolffian duct; this is the iliac artery (Fig. 
16, Il. ar.). Beyond this branch the aorta passes, as the right umbilical 
artery, lateral to the cloaca and the allantois, to the beginning of the body 
stalk, but here becomes obliterated on the right side (Rt. um. ar.). On 
the left side, however, the left umbilical artery (Lt. wm. ar.) becomes a 
much larger channel, and continues along the body stalk. The mesen- 
chyma of the body stalk has been partially dissected away above a certain 
level to show the left umbilical artery and allantois. Throughout the 
first part of its course the umbilical artery is of irregular caliber, but soon 
it becomes smooth. 
Intestine.—The entodermic intestine (Figs. 15, 16, In.) appears as an 
oval tube, with long axis antero-posterior. It is situated in front of the 
