50 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
mandibular portion extends backward and downward in the mandibular arch. Its ventral edge becomes 
indistinguishable from the mesenchyma of this region. A narrow band of crest cells unites the thalamic and 
urtrochlear crests. The facial-acoustic crest (C.fa.-ac.) is completely separated from the trigeminal, but is 
connected by a narrow dorsal band with the glossopharyngeal crest (C.g-ph.). The glossopharyngeal and 
vagus (C.vag.) crests are now completely separated. The posterior end of the latter is continued some 
distance along the spinal portion of the medullary tube. 
The pharynx is somewhat expanded anteriorly. Its floor is fused with the skin ectoderm in 
the oral region, forming the oral plate (Or.pl.). In the drawing the skin ectoderm is represented as cut 
away around the border of this fused area. The thyreoid gland (7%.) appears as a deep pouch from the 
floor of the pharynx, immediately behind the oral plate. Two gill pouches project from the side of the 
pharynx. The first is fused with the overlying ectoderm, which is represented in the figure as left in 
position. In this plate is a slight depression — the beginning of the first gill cleft (@l.e.). The pharynx 
becomes continuous posteriorly with the short broad oesophagus. The yolk stalk (Y.s.) in this embryo is 
connected with approximately the middle half of the gut. The left wall is represented as cut away to a 
great extent, exposing the internal surface of the right wall. The cloaca (Cl.) is represented by a very 
slight dilatation of the hind-gut, and the postanal gut by a short, somewhat constricted segment of the 
tube, connecting the cloaca with the neurenteric canal (Neur.c.). 
This is the first member of the series of reconstructions in which an attempt has been made to 
illustrate the vascular system. The veins of the blastoderm, which are not extensive, unite to form short 
vitelline veins, of which the left is shown in the figure (Y.vit.s.). These remain bifurcated throughout the 
posterior half of the heart but unite anteriorly. The mesothelial layer of the heart is here represented in 
stipple. The ventral mesocardium is broken down, but the dorsal mesocardium (D.mes.) extends the entire 
length of the heart. The ventral aortae, of which the left is shown (Ao.vent.), are separate throughout. 
They are continuous with the first pair of aortic arches (Ao.a.). The aortic arches of the right and left 
sides become confluent dorsally, and from the chamber thus formed, the sinus cephalicus (S.ceph.), there 
extends forward a single vessel. This structure, which can only be followed through a few sections in 
the embryo, is represented as cut off near its origin. It represents the beginning of the vena capitis me- 
dialis (V.cap.med.). Below the sinus cephalicus there is a small dilatation of the aortic arch, which may 
represent the large ophthalmic artery. The dorsal aorta (Ao.dors.) extends backward a little beyond the 
middle of the trunk region. It is irregular in caliber, and in places is apparently solid. 
The notochord (No.) is curved sharply downward at its anterior end. The posterior end is fused 
with the entoderm just anterior to the neurenteric canal. Below the notochord, from the level of the vagus 
crest backward, is seen the subnotocordal rod (Sn.r.). This structure is now larger proportionally than in later 
stages. It is attached to the roof of the pharynx anteriorly, and to the cloaca posteriorly. At several 
points along its free portion, it is enlarged and approaches the roof of the gut, although not in con- 
tact with it. 
Fig. 9. Graphic reconstruction of an embryo 5.2 mm. in length. Normal plate series No. 20. 
(ERRRE@1355.) REEO: 
Only the anterior two-fifths of this embryo is reconstructed. The plan of reconstruction is similar 
to that employed in Fig. 8 except that the otocyst is shown in position, with the skin ectoderm cut away 
around its opening. 
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