A Human Embryo with Seven Pairs of Scmites. 93 
Bonnet in young sheep embryos, both in pericardial and peritoneal 
coelom, the latter only communicating with the extraccelom. 
The pericardial cavity communicates freely with the pleural, 
which in turn passes into the peritoneal ccelom and through it com- 
munication is made on each side with the exocelom at the level 
of somite IV (Fig. 11). ‘There is therefore a complete circuitous 
eanal, by which it is possible to travel from the extraccelom on one 
side through the embryonic ccelom to the extraccelom on the other 
side. 
Heart. The mesodermal walls of the heart are the result of 
confluence, fusion and absorption of the walls of the pericardial 
coelom. The heart is attached dorsally throughout its whole length 
by a mesocardium; ventrally and laterally it is free in the peri 
eardial cavity. The heart roughly fills about one-third of the 
pericardial cavity, is bent upon itself almost at a right angle, with 
convexity to right and coneavity to the left side of the embryo. The 
endothelial heart is a simple tube of flattened cells lying within the 
mesodermal walls but greatly shrunken and collapsed from fixation. 
Nephric system. There is no evidence of the Wolffian bodies or 
ducts. Three independent small cavities are seen in the lateral 
masses of somites I and II on the right side and somite II on the 
left side, none of which connect with celom. These cavities show 
a partially differentiated wall of more or less distinct cubical epithe- 
lium (Fig. 3) and are probably the earliest stages of the pronephric 
tubules. Below somite II the lateral masses become very vague and 
indistinct. Fig. 11 shows a number of cavities, some independent, 
others connecting with the eelom, but agreeing in position with the 
future lateral masses. They are merely small undifferentiated slits 
in the mesoderm, have no definite lining and may or may not be 
homologous with those mentioned above as pronephric tubules and in 
earlier stages of development. They may, however, represent foci 
or ccelomic development, as is suggested by several small spaces 
similar in position and connecting with the ccelom. 
Vascular system. The description of the vascular system of this 
embryo might appear very unorthodox indeed, were it not for the 
previous description by Eternod of a 1.3 mm. human embryo, which 
for a decade has remained alone and unconfirmed, which shows, how- 
