No. 2.] VENTRAL ABDOMINAL WALLS IN MAN. 361 
umbilicus and pubis is relatively much shorter than it is in the 
adult. The distance between the end of the sternum and 
the umbilicus is twice the distance between the umbilicus and 
the pubis in embryos of about eight weeks, while in the adult 
these two measurements are equal.! Before the two pubic bones 
have met to form the symphysis, this difference is still greater. 
This narrowed area between the pubis and umbilicus in the 
early embryos may possibly account for the blending of the 
three muscle segments of the rectus in this region. The appear- 
ance in embryos XLIII and XXII, Figs. 4 and 6, also indicate 
this. 
Shifting of the Milk Line in Pigs’ Embryos.— The inva- 
sion of the membrana reuniens by the rectus is also indicated 
by the shifting of the milk line. O. Schultze? has shown that 
the mammary glands in pigs arise as a ridge of ectoderm on 
either side of the body immediately to the ventral side of the 
Wolffian ridge. In a short time the milk ridge breaks up into 
islands, each of which marks a later mammary gland. 
Figs. 9, 11, and 13 show the position of the milk line in pigs’ 
embryos, giving the relative positions of the line in the three 
different stages. Fresh specimens show clearly that the 
abdominal walls are extremely thin on the ventral side of the 
line, while on the dorsal side they are thicker and apparently 
finished. Sections of the three embryos pictured in Figs. 9, 
II, and 13 are shown in Figs. 10, 12, and 14. The sections 
show that the line lies immediately over the rectus, and from 
this we can locate the position of the rectus in any embryo 
before it is cut. The wandering of the rectus is indicated then 
in pigs’ embryo by the shifting of the milk line. 
Development of the Internal Mammary and Deep Epigastric 
Arteries. — Before the intercostal vessels are formed there 
appears to be a circulation through the membrana reuniens, as 
is indicated to us by a figure given by Kolliker,? as well as by 
the description of His. Kolliker pictures a cow’s embryo with 
1 See also the figures given by Merkel, Adhandl. d. k. Ges. d. Wiss., Gottingen, 
ee Schultze, Azat. Anz., Bd. vii,and Verhandl. d. Phys.-med. Ges., Wiirzburg, 
Bd. xxvi. 3 Kolliker, Grundriss, p. 103, Fig. 58. 
4 His, Anat. Mensch. Embryonem, Bad. iii, p. 206, also Fig. 130. 
