No. 408.] STUDY OF MAMMALIAN EMBRYOLOGY. 933 
and the base of the umbilical cord. The two umbilical veins 
enter the liver directly from the cord. The right vein (V.U.D.) 
is already smaller than its left fellow (V.U.S.). They are con- 
nected respectively with the right and left ventral lobes of the 
liver. The right vein in a little later stage is no longer rec- 
ognizable as an open channel. It will be noticed that the 
right and left sides of the abdominal cavity (splanchnoccele) 
are completely separated from one another, and that there is a 
special part of this cavity shown in the section between the 
stomach and the right dorsal lobe of the liver, and which is 
known as the lesser peritoneal space or cavity of the omentum. 
The body-wall (Som.), or somatopleure,! consists of three 
layers, — the ectoderm (Zc.), the mesenchyma (mes.), and the 
mesothelium (zzs24.). It is of the greatest importance for the 
student to understand the arrangement of the germ-layers in 
the somatopleure. The mesothelium (mst%.) is commonly 
known in the descriptive anatomy of the adult as the peri- 
toneal epithelium. In sections like that of Fig. 7 it can be 
followed not only over the inner surface of the body-wall but 
over the surface of the Wolffian body and liver, and upon the 
left side of the body also over the surfaces of the greater 
omentum, stomach, and lesser omentum. We see, in fact, 
that the body cavity is completely bounded by mesothelium, 
and that all the abdominal viscera are therefore outside of it. 
This conception, which is so important yet so difficult to the 
student of anatomy, is easily mastered by the study of the 
embryonic relations. 
From the sagittal series of the same stage many instructive 
pictures are obtainable. I have selected a median section of 
the head, and one passing through the principal cephalic 
ganglia, for engraving, and give here the latter. The mag- 
nification is 22 diameters, the same as for the transverse sec- 
tions Figs. 2 to 7. To the student of anatomy such a sec- 
tion as is shown in Fig. 8 is highly instructive, for it exhibits 
in a single picture many important fundamental relations of 
the cephalic nerves, particularly of the second, fifth, seventh, 
1 It is much to be regretted that German embryologists use the term **somato- 
pleure " erroneously. 
