Normal plates of the development of the rabbit (Lepus cuniculus L.). 7 
The more important changes in this ıı-day embryo as compared with that of 10'/, days are the 
following: the neck bend is more developed; the brow overhangs the heart chamber more, so that there 
is less space between the tail end of the body and the brow; the caudal end of the body is not rolled over 
to the right so much but makes nearly a half turn; nasal pits are now visible, as well as the openings of 
the lentic vesicles; the cervical sinus is more distinct; the third and fourth gill arches and the third external 
gill cleft are plainer; the limb buds are a little larger. 
Fig. 26 (X 5). 
Embryo removed from uterus Iı!/, days after coitus. ZENKER fixation. Measured 6.0 mm. longest 
diameter: vertex-brow, 3.0 mm; vertex-neck, 3.6 mm. The brow projects beyond the heart chamber so far 
as to leave only a short interval between itself and the region of the hind limb buds. The head bend forms 
a wide acute angle. The neck bend forms an obtuse angle. There is a slight prominence in the region 
of the mid-brain. The trunk is bent to approximately a right angle, but in the form of a gradual curve, 
at a point a little caudad of the fore limb buds. The tail end of the body bends to the right to the extent 
of nearly half one turn. The tip of the tail is somewhat straightened. The lentic vesicles are closed. The 
thin, expanded, kite-shaped roof-of the fourth ventricle is distinct, as also the trigeminal ganglion. The 
nasal pits are rather shallow. The maxillary process, mandibular and hyoid arches are prominent. The cervical 
sinus is distinct. In it appear the third and fourth gill arches. The first and second external gill clefts are 
clearly marked; the third less so, but distinct. The heart chamber forms a projection under the mouth 
region. The auricular region, ventricular limb and truncus arteriosus of the heart are clearly distinguishable. 
The fore and hind limb buds project. Externally there are visible approximately 37 or 38 myotomes 
(segments). Compare, for internal development, Table No. 13. 
The more important changes in this ı1!/,-day embryo as compared with that of 11 days are the 
following: the head bend makes more plainly a wide acute angle; the neck bend is more developed so as 
to approach more nearly a right angle; a slight prominence in the region of the mid-brain has appeared; 
the portion of the trunk between the fore and hind limb buds appears a little straighter and longer; the 
lentic vesicles are now closed: the nasal pits are deeper; the cervical sinus appears more depressed; the 
fore and hind limb buds are a little larger. 
Fig. 27 (X 5). 
Embryo removed from uterus I2 days after coitus. ZENKER fixation. Measured 6.0 mm. longest 
- diameter: vertex-brow, 3.2 mm.; vertex-neck, 4.0 mm. The brow projects beyond the heart chamber upon 
which the region between the nasal pits appears to rest. The head bend is an acute angle. The neck bend 
is an obtuse angle. The main bend of the trunk caudad of the fore limb buds forms an obtuse rather 
than a right angle. The caudal end of the body bends over to the right making at least half of one turn 
The hemispheres are visible. The nasal pits are wide open. The roof of the fourth ventricle and trigeminal 
ganglion are conspicuous. ‘The maxillary process, mandibular and hyoid arches are large and prominent. The 
cervical sinus and therein the third and fourth gill arches are distinct. The first, second and third external gill 
clefts are plain. The auricular and ventricular regions of the heart are easily visible through the pericardial 
wall. The fore and hind limb buds project: the latter are on the upper part of the caudal turn ofthe body. 
Externally there are visible approximately 40 myotomes. Compare, for internal development, Table No. 14. 
This 12-day embryo shows no very important changes as compared with that of ıı!/, days; the 
main differences are the following: the main bend of the trunk caudad of the fore limbs has straightened 
