64 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
vessel (A.eff.hyo.) except for a large number of capillary loops which extend into the gill filaments. For the 
sake of clearness these have been omitted in the figure. The third and fourth aortic arches present. 
characteristic stages in the differentiation of these structures and may be described together. The third 
the afferent artery still possesses dorsal and ventral communications with the efferent vessel, but these 
communications are very much reduced in size and their lumina are almost occluded. The efferent artery 
is divided into a posterior and anterior limb. These two vessels are connected by three large irregular 
vascular channels which lie mesial to the afferent vessel. The stubs of the vascular loops of these 
demibranchs have been represented in outline. The efferent portion of the fourth aortic arch is 
similar to that of the third with the exception that the ventral communication with the afferent vessel is 
represented only by a short sprig in which the lumen is occluded, and that there are two instead of three 
communications between the anterior and posterior portions of the afferent artery. The fifth aortic arch 
represents a somewhat earlier stage in the development of the branchial arterial system. Both the afferent 
and efferent vessels form complete connections with the ventral and dorsal trunks of the arch although the 
dorsal connection of the afferent vessel is considerably reduced in size. The afferent portion is divided 
into anterior and posterior sections after the same manner described for the two preceding arches. The 
afferent portion of the sixth arch has retained its original size throughout and is complete. The efferent 
portion is represented by a vessel springing from the dorsal end of the arch extending ventrally anterior 
to the afferent portion and communicating with the ventral root by a very small channel. From this 
afferent portion there extend posteriorly, mesial to the efferent trunk, several irregular sprouts which 
are doubtless destined to form the last branchial artery. That section of the dorsal aorta Iying between 
the second and third arches is much reduced in caliber, and the connection between the first arch and 
the dorsal aorta, i. e., the sinus cephalicus, is somewhat obscured in the figure by the overlapping nerves. 
It is very much enlarged and from it the ophthalmic artery (A.oph.) extends forward mesial to the optic 
cup. The first of the visceral arteries, the coeliac artery (A.coel.), is now established. It arises by two 
segmentally placed branches from the aorta just posterior to the pancreas and extends a short distance both 
forward and backward in the mesentery. 
From the sinus venosus there spring three sets of vessels. Dorsally there extends a short thick 
common cardinal (V.card.com.) which divides almost immediately into the anterior and posterior cardinals. 
The lateral vein (V.lat.) opens below the common cardinal vein. The opening on the anterior surface of 
the lateral vein is that of the jugular vein. 
Fig. 17. Graphic reconstruction of the head somites of an embryo 4 mm. in length. Normal plate 
series No. 16. (H.E.C. 430.) X 33. 
The general method of representation which has been employed in this figure is the same as that 
used particularly for the mesodermic structures in Fig. 6. The embryo is represented as sectioned trans- 
versely a little behind the anterior wall of the yolk stalk. The general description of structures other than 
the mesoderm is given in connection with the description of Fig. 7 which is another reconstruction of the same 
embryo. The mandibular somite (So.mand.) consists of a large distal chamber extending forward above the 
eye and lying lateral to the pharynx and mid-brain. It is connected with the general lateral mesoderm by 
a short broad stalk. The small premandibular somite (So.prem.) lies mesial to the ventral portion of the 
mandibular somite. It is connected anteriorly by a short broad stalk with the anterior somite (So.ant.). 
This body is definitely formed and possesses epithelial walls dorsally, but it is mesenchymous ventrally, 
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