930 THE AMERICAN NATURALIST. [VoL. XXXIV. 
(Val.) is well advanced. The dorsal or trunk region of the 
section is formed chiefly by the more or less dense mesen- 
chyma. It includes important structures. The spinal cord 
(Sp.c.) resembles that in Fig. 5, but is both larger and more 
differentiated. The fundamental morphological characteristics 
of a spinal nerve are well illustrated by the left nerve of Fig. 6. 
The dorsal root (D.R.) bears the ganglion (G.) and is joined a 
little below the level of the cord by the ventral root (V. A.) to 
form a single nerve trunk, which, however, soon subdivides into 
its two primary branches; the first or dorsal branch, ramus 
dorsalis (R.D.), bends at an acute angle upwards and outwards; 
the second or ventral continues downward and curves into the 
limb; owing to this curvature it must be traced through adja- 
cent sections, but in Fig. 6 it can be seen again (A. V.) at the 
base of the limb, where it crosses the subclavian vein (Sc/.V.). 
Some distance below the spinal cord is the small notochord 
(VcÀ.). Much lower, and between the two ducts of Cuvier, 
appear the cesophagus (Oe.) and trachea (772.), each a ring of 
entodermal epithelium with commencing condensation of sur- 
rounding mesenchyma. This condensed tissue is the anlage 
of the future submucosa and muscularis. Above the cesoph- 
agus, to the right and left, appears the dorsal aortic trunk, 
of which the left only (Ao.S.) is completely retained through- 
out life. It should be noted that the blood vessels, including 
the largest, have at this stage only endothelial walls, the 
adventitial and muscular coats being added later. A// blood 
vessels are endothelial chambers, and this fundamental con- 
ception ought to be made clear to the student. 
We now pass to a section (No. 633) well below the heart, in 
order to study the characteristics of the Wolffian body, stomach, 
and liver., At this level, as comparison of the figures will 
show, the body of the embryo has its greatest dimensions. 
The upper edge (Us) of the umbilical cord also appears in 
this section. The spinal cord, with the ganglia and nerves, 
presents essentially the same features as in Figs. 5 and 6. 
The notochord (Nch.) forms a small circle in section and is 
surrounded by an area over which the mesenchymal cells are 
more crowded or condensed than elsewhere. The condensed 
