918 THE AMERICAN NATURALIST. [VoL. XXXIV. 
relatively straight. It is thus brought about that the dorsal 
side of the embryo is two or three times as long as the 
ventral. From the ventral side springs the large umbilical 
cord, the connection of which with the body occupies prac- 
tically the entire length of the ventral median line of the 
abdominal region proper. Above the umbilical cord the pro- 
tuberant outline of the cardiac region passes below the 
nasal (V.) and mandibular (Md.) regions toward the cervical 
sinus (C.S.). The long tapering tail extends near the umbilical 
cord. 
The surface modeling of the embryo offers important fea- 
tures. Beginning with the head, we observe first the shallow 
depression, constituting the nasal pit (/V.). The eye (Op.) is 
entirely without lids; the lens appears in the center and is sur- 
rounded by the outlines of the optic vesicle. The small size of 
the eye is a characteristic of the mammalian embryo ; by which 
it differs from all sauropsidan forms, but in certain other mam- 
mals the embryonic eye is slightly larger than in the pig. 
Below the eye is the maxillary process (JZx.), which is destined 
to form the greater part of the upper jaw; the anterior bound- 
ary of the maxillary process is marked by a shallow depression, 
the lachrymal groove, which runs from the angle of the eye 
(Op.) to the nasal pit (W.). The mandibular process (Md.), out 
of which the lower jaw is developed, is bounded in front by a 
groove separating it from the maxillary process, and behind by 
a second groove (Aw.), the anlage of the future meatus audi- 
torius externus. This groove marks the boundary between the 
mandibular process and the first or hyoid branchial arch, and 
is itself the ectodermal member of the first gill-cleft. The 
fourth ventricle ( Vez.) or cavity of the hind-brain, having very 
thin walls for its roof, can be readily distinguished. _ The 
thickened floor of the fourth ventricle is the anlage of the 
medulla oblongata. The cervical sinus (C.S.) is an area of 
invagination, presenting at this stage a triangular outline ; 
within the sinus are found the external or ectodermal ter 
minations of the second and third gill-clefts. The territory 
of the mandibular process and cervical sinus corresponds to 
the pharyngeal region. It is the site of some of the most 
