358 MALL. [Vou. XIV. 
rectus is innervated by a single intercostal nerve and, therefore, 
must be viewed as belonging to the same myotome: 
The lateral position of the rectus, which has already been 
noted by Kélliker,! is also easily seen in transverse section, as 
shown in Fig. 5. Unfortunately, the stages between XLIII 
and II are missing, but the lateral position of the rectus does 
show that a finished rectus is formed at a very early date, and 
that it must still shift one-half the distance around the body 
in order to obtain its final position. The muscle is nearer the 
middle line at its upper end than at its lower, due no doubt to 
the more rapid growth of the ribs than of the pubis, to which 
it is now attached. 
In a later stage, Fig. 6, we have the same condition of things 
as in No. XLIII, only more advanced. In this reconstruction 
it was impossible to make out definitely all the serration of the 
external oblique and the segments in the rectus, because the 
sections were transverse to the embryo, while in XLIII they 
were sagittal. The transverse sections (Figs. 7, 8) show the 
exact position of the rectus at its different levels. 
The two recti reach the middle line above the umbilical cord 
about the eighth week, while the main bulk of the intestine is 
still in the cord. At this time the ventral abdominal walls 
above the umbilicus are completed, and their further growth is 
simply a matter of expansion. About the end of the ninth 
week the intestine is withdrawn from the cord, the opening 
soon becoming closed by an adhesion of the edges of the periph- 
ery of the communication of the coelom of the cord and the 
abdominal cavity. As soon as this is accomplished the stem 
of the cord is surrounded by a membrane about 5 mm. in 
diameter, which in turn is encircled by the two recti. It is 
very easy with a probe to reéstablish the old communication, 
and in so doing the membrane again returns to its former posi- 
tion around the end of the cord, showing by this reverse process 
its origin. Very soon after this membrane is formed the recti 
invade it also, thus completing the abdominal walls at this point. 
Relation of the Segments of the Rectus to the Umbtlicus. — 
Judging by the illustrations in the various atlases and text-books 
1 KOlliker, Grundriss, 1884, p. 340. 
