series of figures. The scales,
especially along the mid-dorsal line, are shown as an area of less
closely dotted tissue.

The lungs, _lu_, cut here through their anterior ends, are large, but do
not nearly fill the cavities, _bc_, in which they lie; they have the
sacculated appearance characteristic of embryonic lung tissue.

The oesophagus, _oe_, is cut through about its middle region, where its
caliber is greatest. As was said above, its dorso-ventral diameter is
more than twice its lateral diameter, caused partly by the oblique angle
at which it was cut. Its wall, figure 7H, is very thin and exhibits a
dense layer of mesoblastic tissue, in which circular and longitudinal
muscle layers are beginning to differentiate. It is lined by an
epithelium which here consists of a single layer of columnar or cuboidal
cells with large nuclei. On the ventral side, where the oesophageal wall
is in contact with that of the trachea the epithelium is somewhat
thickened by an increase in the number of cell layers. With the low
magnification used these details could not, of course, be shown.

The trachea, _ta_, is of much smaller caliber than the oesophagus,
especially in its dorso-ventral diameter. While its epithelial lining is
not yet appreciably different from that of the oesophagus, its
connective tissue wall is much thicker and shows numerous condensations,
the rudiments of the cartilaginous rings. In the region represented by
this figure the connective tissue layers of the trachea and oesophagus
are continuous with each other, but cephalad and caudad to this point
they are distinct, though sometimes in contact. Several large blood
vessels, _bv_, on each side of the oesophagus probably represent the
carotids and jugulars, but they were not worked out to determine with
certainty which they were.

Eighty-five sections (figure 7, _X_) caudad to the one under discussion
the trachea divides into the two bronchi. These bronchi gradually
separate from each other until, at the point at which they open into the
lungs, about eighty sections caudad to their point of separation, they
lie on either side of the ventral third of the oesophagus.

Figure 7B represents a section through the plane 480 of figure 7. The
section is just cephalad to the heart, and passes through the caudal
third of the lungs, _lu_, which have the same appearance as in the
preceding figure; also through the extreme cephalic end of the liver,
_li_. The lungs here mu