No. 2.] DEVELOPMENT OF THE HUMAN COELOM. 425 
it. Under these conditions the simple weight of the vesicle is 
sufficient to bend the back of the embryo as pictured by His. 
To return to the coelom. At the hinder end of the embryo 
the coelom dips into the body overlapping the hind-gut in the 
neighborhood of the neurenteric canal, as shown in Fig. 20. 
This cavity communicates with its fellow on the opposite side 
through an opening between the umbilical vesicle and the 
allantois, marked O in Fig. 16. This communication has 
already been described by His! for an embryo somewhat older. 
If, now, the point O in Fig. 16 is approximated towards NC, 
with a flexion of the embryo at the same time, this communi- 
cation is easily explained. In other words, as the hind-gut is 
being separated from the umbilical vesicle, a groove-like por- 
tion of the coelom is also included in the body of the embryo. 
At the hinder portion of the embryo, on either side, the 
coelomic grooves extend deeper into the body of the embryo, 
and communicate with each other around the aboral side of the 
stem of the umbilical vesicle. This communication is shown 
well by His in Fig. I, B, Plate VI of his AZ/as, as well as in the 
same figure, page 299 of Minot’s Embryology. Excellent profile 
views showing this point are given in all the embryos figured 
on Plate IX of His’s Addas. 
I emphasize this point in order to exclude the ventral mesen- 
tery for this portion of the embryo. The fact that this 
mesentery could never have existed in the human embryo is 
also proved by a careful examination of His’s models of human 
embryos made by Ziegler. 
As we pass towards the head in embryo XII the coelomic 
groove communicates freely with the extraembryonic coelom 
until the region of the membrana reuniens is reached. This is 
shown in Fig. 19, MR, with the membrana reuniens complete 
on one side, but not yet united on the other. The membrana 
reuniens extends up to the heart, and separates the pericardial 
cavity from the extraembryonic coelom, then crosses the ven- 
tral median line to return on the opposite side of the embryo. 
Throughout the extent of the membrana reuniens there is a 
great increase of mesodermal tissue, which encircles completely 
1 His: Anat. mensch. Embryonen, I, p. 126. 
