166 STUDY OF PIG EMBRYOS. 



pancreatic anlages. Similarly only the entodermal portion of the trachea and 

 lungs is included, and the same is true of the caudal end of the Wolffian duct 

 and of its outgrowth, which forms the anlage of the kidney. The same is further 

 true of the gall-bladder, of which only the epithelial portion is represented. In 

 this figure the arterial system is fully displayed. The pulmonary artery and the 

 aortic trunk are completely separated. A small artery from the pulmonary 

 arch to the lungs is included, and the figure shows the entire system of branches 

 from the main aorta. 



The development of the aortic arches requires a few words of special ex- 

 planation. The disposition of the vessels is perhaps more clearly shown in figure 

 1 02. In an earlier stage the heart lies immediately under the pharynx and gives 

 off from its aortic end the short aortic trunk which runs upward toward the floor 

 of the pharynx. From the aortic trunk spring five pairs of vessels, known as the 

 aortic arches (Fig. 102). Ten vessels are symmetrically placed, five on each side 

 of the pharynx. They pass around the pharynx dorsalwards. The five vessels 

 of each side very soon become united by a dorsal longitudinal trunk the descend- 

 ing aorta, which passes down through the cervical region of the embryo and 

 meets its fellow at about the level of the diaphragm, unites with it, and thus 

 forms the single median dorsal aorta. The first arch very soon disappears (Fig. 

 103). Its prolongations with the vessels on the ventral side run forward to the 

 lower jaw and give rise to the external carotid. The vessel on the dorsal side also 

 persists and gives rise to the internal carotid. Presently the second arch also 

 disappears, and both carotids are, as it were, thereby lengthened. This is the con- 

 dition which we find in our embryo of 12 mm. (Fig. 101). The third, fourth, and 

 fifth arches are still present. From the base of the third arch runs forward the 

 external carotid, and from the summit of the third arch runs forward the internal 

 carotid. The dorsal ends of the third and fourth arches are still connected, but 

 this connection, instead of being a large aortic vessel, as in earlier stages, has now 

 contracted and almost disappeared, and will soon be lost altogether, so that in 

 the adult there will be no connection between the dorsal ends of the third and 

 fourth arches. The fifth arch is still connected with the dorsal end of the fourth. 

 It gives off the small pulmonary artery to the lungs. On the side toward the 

 heart the relations of the arches are also changed. The main aortic vessel which 

 springs from the heart is, in the 12 mm. pig, divided into two vessels — the pul- 

 monary aorta on the ventral side and the true aorta in a more dorsal position. 

 The division has so taken place that the third and fourth arches are connected 

 only with the true aorta, while the fifth arch is connected only with the pulmo- 

 nary aorta. The part of the fifth arch on the left side between the origin of the 

 pulmonary artery proper and the main descending aorta offers at this stage an 



