8 Normentafeln zur Entwicklungsgeschichte der Wirbelthiere. 
a little so that it forms an obtuse rather than a right angle; the nasal pits are deeper; the umbilical cord 
has become a somewhat more definite structure. 
&) Fi: 8; 
Embryo removed from uterus 12!/, days after coitus. ZENKER fixation. Measured in longest diameter 
7.6 mm.: vertex-brow, 4.0 mm; vertex-neck, 4.0 mm. The end of the head projects well beyond the heart 
chamber. The head bend is a wide acute angle. The neck bend is nearly right-angled. The trunk, between 
the fore and hind limbs, bends gradually within the extent of an obtuse angle. In the region of the hind 
limbs the caudal end of the body is bent straight up and then turns to the right. The tip of the tail is 
bent a little outward away from the body: it is not far from the brow. The nasal pits are open. The 
outlines of the hemispheres are visible in front of the eye. There is a slight prominence in the region of 
the mid-brain. The eye, with the lens, is conspicuous. The thin roof of the fourth ventricle is evident. 
The maxillary process, mandibular and hyoid arches are large and prominent. There are on the mandibular 
and hyoid arches small irregularities, the beginnings of the tubercles of the external ear. The cervical 
sinus is marked only by a small hole. No third and fourth gill arches are recognizable. The first or auditory 
gill cleft is nowhere closed. No other gill cleft is clearly distinguishable. The auricles and the ventricular 
region of the heart are easily seen externally. The fore and hind limb buds project quite prominently. 
The fore limb bud shows a beginning division into two parts, an outer, broader and an inner, narrower and 
rounder part. The outer part is curved on its edge, but flattened from side to side: it is the beginning ot 
the manus, The hind limb bud shows no sign of division: it is, however, curved on its edge and flattened 
from side to side distally. There is a quite distinct umbilical cord. Externally there are visible from the neck 
to the tip of the tail approximately 47 to 48 myotomes. Compare, for internal development, Table No. 15. 
The more important changes, some of which are quite striking, in this 12!|,-day embryo as compared 
with that of 12 days, are the following: the heart chamber appears relatively prominent; the neck bend 
has become nearly a right angle; the trunk has straightened out considerably; the caudal end of the body 
is bent upright rather than in the form of a turn; the openings of the nasal pits are somewhat narrower; 
the trigeminal ganglion is not so prominent; the tubercles of the external ear begin as described above; 
the cervical sinus is reduced to a small hole; the third and fourth gill arches are no longer recognizable; no 
gill cleft is distinguishable except the auditory; the limb buds are larger, the fore limb showing the beginning 
differentiation described above; the hind limbs now mark the point of the upward caudal bend instead of 
being at the top of this bend, and are more nearly in a direct line with the fore limbs. 
122 ’ 
2 Fig. 29 (X 5). 
Embryo removed from uterus 13 days after coitus. ZENKER fixation. Measured in longest diameter 
9.8 mm.: vertex-brow, 5.0 mm.; vertex-neck, 5.0 mm. The brow projects so far that the anterior nares face 
inward toward the pericardial wall, to which they are very near. The head bend and neck bend are both 
approximately right-angled. The main bend of the trunk caudad of the fore limbs forms a wide obtuse 
angle, gradually curving. The caudal end of the body is bent straight upward and a little to the right in 
the region of the hind limbs. The tip of the tail, which is turned away from the umbilicus, nearly touches 
the brow. The outlines of the hemispheres are clearly visible. There is a slight elevation in the region 
of the mid-brain. The eye is prominent, the lens easily recognizable. The thin expanded roof of the fourth 
ventricle is evident. The trigeminal ganglion is not at all clear. The maxillary process is prominent. The 
tubercles of the external ear on the mandibular and hyoid arches are well marked. These two arches meet 
