413 



development. In the light of observations on pig embryos, and of the 

 descriptions of the fifth arch by Zimmermann and Tandlek in rabbit, 

 rat, and human embryos, I have not hesitated to interpret these 

 structures as rudiments of the fifth aortic arch. 



The entodermal pouch between the fourth and sixth arches is un- 

 usually broad and a very small portion of it appears to be partly 

 constricted oif between these two vessels. There are not, however, 

 in the rabbits observed, two distinct entodermal pouches in this region, 

 as my observations show to exist in the pig embryos. 



Embryo of twelve and one-half days. Fig. 6 shows 

 a reconstruction of the aortic arches of the left side of an embryo 

 twelve and one-half days old. There are present in this stage, three 

 pairs of complete aortic arches, the third, fourth and sixth, and, also, 

 a pair of extremely small spurs from the aortic roots in the hyoid 

 visceral arch. The complete vessels are of nearly uniform size and 

 about equally distant from one another. From the sixth, on each side, a 

 pulmonary artery has started. In its development it agrees with the 

 descriptions of His, Zimmermann and Bremer and not with that of 

 Rathke. 



Details in reference to the development of carotids and sub- 

 clavians, which are given in my complete paper, have been omitted 

 here. 



B. The Aortic Arches in Pig Embryos. 



Embryo of the twentieth day (early). The pig and 

 rabbit series overlap. The youngest pig embryo reconstructed was 

 one in the early part of the twentieth day of development. Its 

 aortic arches of the right side are shown in Fig. 7. 



Two complete arches, the third and fourth, are present, but 

 portions of the first, second and sixth arches also exist. The first 

 and second are in the process of degeneration, but the sixth is not 

 yet fully formed. The first arch is represented by a dorsal rudiment 

 (see Fig. 7) but the second arch is more nearly complete. A dorsal 

 rudiment slightly larger than that of the first arch leaves the dorsal 

 aorta as a single vessel, but, after a short distance its channel becomes 

 divided into several smaller ones. The latter merge into a small 

 vessel which terminates a short distance above the large forward ex- 

 tension of the truncus arteriosus. From the anterior wall of this 

 prolongation of the truncus, a slender and short vessel extends up- 

 ward into the hyoid arch. Between this ventral rudiment of the 

 second arch and the larger dorsal rudiment, lie numerous small dis- 

 connected pockets in the mesoderm. 



