52 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



PLEXIFORM ORIGIN OF ARCHES. 



The opponents of the theory of a plexiform origin of the blood-vessels have 

 pointed to the aortic arches as an unassailable example of the correctness of then- 

 view. Lewis and others have, however, placed beyond doubt the preexistence of a 

 vascular net. The plexus from which the aortic arches develop may cover a wide 

 field or may be restricted, depending upon the amount of mesenchymal territory 

 available. In the case of the second, third, and fourth aortic arches, this is limited 

 by the small cross-section of their visceral arches. The plexuses preceding the 

 first and pulmonary arches are not so restricted and also have other distinctive 

 features. 



The first arch was shown by Lewis (1904) to arise in rabbits from an angio- 

 blastic net in company with its ventral connections and the primitive aortse. This 

 was confirmed by Bremer (1912). Evans (1909a) has demonstrated by injection 

 the capillary net preceding it in the duck. In the youngest human embryo of our 

 series (1.3 mm. long) the first arch, in its irregular course and in the presence of 

 islands, still gives evidence of its origin from a net. The manner of development of 

 the second, third, and fourth arches is well illustrated in our material, though 

 the series is not complete for any but the second. One of the first indications of 

 the development of an arch is a slight expansion of the dorsal aorta down into the 

 visceral arch. A similar but more marked projection is seen at the same time 

 pointing caudally and laterally from the common ventral chamber from which the 

 arches arise. This will be termed, for reasons which will be explained later, the 

 aortic sac. 



An early stage in the formation of the second arch has recently been studied by 

 Dr. C. L. Davis 1 in a 20-somite embryo. Angioblastic cords and capillaries extend 

 down from the dorsal aorta on one side (plate 1, figs. 29 and 30, drawn from Dr. 

 Davis's models), while on the other an open channel leads ventrally through the 

 arch for a short distance and then goes over into the primitive net. There is also 

 a vessel (not shown in the figures) which extends up from the aortic sac into the 

 visceral arch and ends in the net. Models of three embryos, of stages ranging from 

 4 to 17 somites, show beautifully the process somewhat farther along. In two of 

 these a projection from the aorta extends down nearly to the sac, where it ends 

 in capillaries and angioblastic cords. In the other the chief projection is from the 

 sac. It extends upward nearly to the aorta and is separated by a plexus from a 

 short downward-directed sprout arising from the aorta. The appearance of a large 

 channel so soon after the outgrowth of a sparse net is not readily explained as 

 entirely the result of a working over and proliferation of the endothelium of the net. 

 It seems more probable that the development in part takes the form of an outgrowth 

 of the bulging, so that the artery sends out a sprout to supplement the growth 

 activity of the net. 



A 4-mm. embryo (No. 836) shows the third arch just completed. It is still 

 irregular in caliber and tortuous. As it enlarges, however, as seen in other em- 



1 Through the kindness of Dr. Davis I have had an opportunity to read his finished manuscript and to examine his 

 models and drawings. 



