Normal plates of the development of the rabbit (Lepus cuniculus L.). 5 
visible through the thin wall of this chamber. The fovea cardiaca or entrance to the fore-gut is very distinct. 
The ventral bend of the head begins just posterior to the heart, or in the region of the fovea cardiaca. 
The mesodermal allantoic fold is prominent. Compare, for internal development, Tables Nos. 6 and 7. 
As compared with the 8!/,-day embryo, this 9-day stage shows the following more important 
changes: there are ten distincet segments instead of eight; the head now bends ventrad forming a decided 
angle with the body; the proamnion surrounds the head; the caudal amnion is of much greater extent; the 
medullary tube is closed nearly throughout the body; the otic pits are visible; the heart chamber begins 
to project. 
> Figs. 10 (X 10), 22 (X 5) and 22a (X 5). 
Embryo removed from uterus g!/, days after coitus. ZENKER fixation. Measured in a direct line 
from head to tip of tail 3.4 mm.; from head to bend of body 2.4 mm.; from bend of body to tip of tail 
2.4 mm. The head of the embryo is bent at a point corresponding to the mid-brain approximately to a 
right angle. The body, also, at a point just posterior to the heart is bent to the same degree as the head. 
The caudal end of the body is twisted around sharply to the right so that it appears folded over the ad- 
joining region. The optic vesicles are distinctly visible. There is a slight expansion of the roof of the hind- 
brain. The openings of the otic cups are distinct. The bent heart tube is visible through the thin pericardial 
wall. The mandibular and hyoid arches are clearly marked; the maxillary process barely so. Segments 
are visible externally to the number ot nearly twenty-three. The caudal segments are not easily counted 
with accuracy on external observation. Compare, for internal development, Tables Nos. 8 and 9. However, 
Table No. 8 is a stage a little younger; Table No.9 a stage a little older than the embryo described above, 
As compared with the 9-day stage, this g!/,-day embryo shows the following more important 
changes: the head bend at the point of the mid-brain is clearly developed to the extent of a right angle; 
the bend of the body just caudad of the heart is now approximately right-angled instead of obtuse; the 
caudal end of the embryo is twisted over to the right; the optic vesicles are more distinct; the otic cups 
are deeper, though still open; the heart is more prominent; the mandibular and hyoid arches have appeared, 
as well as a suggestion of the maxillary process; the segments are more numerous. 
© Figs. ıı (X 10), 23 (X 5) and 23a (X 5). 
Embryo removed from uterus Io days after coitus. ZENKER fixation. Measured 3.8 mm. longest 
diameter viz. from vertex!) of head to the most remote point on body; from vertex to brow 1.6 mm. The 
head of the embryo is bent to an acute angle; the body is bent in two places approximately to a right 
angle, while the tail end is twisted over sharply to the right. The optic vesicles are conspicuous. The 
thin expanded roof of the hind-brain has acquired a kite shape. Alongside of the hind-brain, the trigeminal 
ganglion is clearly visible; the otocyst, perhaps both the otocyst and acustico-facial ganglion, is just dis- 
tinguishable The oral cavity is wide open. The maxillary process, mandibular and hyoid arches are dis- 
tinct. There are clear evidences of a third arch posterior to the hyoid. The first and second external gill 
clefts are plain. The mandibular arches do not meet at the surface in the mid-ventral line. The bent heart 
tube is clearly visible through the thin pericardial wall. A slight thickening at the bend of the cephalic 
end of the body marks the beginning fore-limb bud. Near the tail, another slight thickening marks the be- 
ginning hind limb bud. Segments are visible externally to the number of thirty, approximately. Compare, 
for internal development, Table No. 10. 
I) Vertex is here used in the sense of the German “Scheitel” (top). 
