Embryonic history of tlie aortic arches in mammals. 405 



formation. Its absence at the close of the tenth day (Fig. 6) and 

 its presence as a complete vessel at the close of the eleventh 

 indicates however, that it is formed during the interval. The vessel 

 lying between the fourth and sixth arches (Fig. 8 x) and the 

 evagination of the dorsal aorta described in connection (Fig. 8 y) 

 bear the relation to each other that exists in a better developed 

 condition of the supposed fifth aortic arch in an embryo of eleven 

 and one-half days. The ventral element of the sixth arch in the 

 embryo under consideration is the first to make its appearance. The 

 subclavians probably form during the eleventh day, and arise from 

 the aortic trunk as Hochstettee has described. 



Embryo of eleven and one- half days. The external 

 anatomy of the embryo is shown in Fig. 9, Plate 22. Four well 

 marked visceral arches are present. The mandibular arch is divided 

 by a deep, almost median fissure into two lobes, and the hyoid arch 

 shows a transverse depression in its middle region. As in the 

 preceding embryos, the third and fourth visceral arches are relati- 

 vely small. 



Fig. 10, Plate 23 shows a reconstruction of the aortic arches of 

 the left side. The anterior prolongation of the dorsal aorta divides 

 into two small vessels which supply the regions of the brain. A 

 ventral remnant apparently of the first arch is present, but it has 

 the character of a sinus, and no actual connection with the second 

 arch could be found. This condition is practically identical for the 

 two sides. Unfortunately a few sections passing through the second 

 arch and dorsal aorta were destroyed, but the course of these 

 vessels can be determined with but slight doubt from the remaining 

 sections. A distinct vessel passes downward from the dorsal aorta,, 

 through the hyoid arch. A ventral remnant of about equal length 

 takes an upward course from the truncus arteriosus to the level of 

 the dorsal branch, but ends a short distance in front of it, instead 

 of directly underneath. The third arch is still a large vessel, but 

 is not relatively of as great size as in the preceding embryo. The 

 fourth arch has increased in size and nearly equals the third in 

 diameter. Its walls have also become continuous. 



In the space between the fourth and pulmonary arches, structures 

 are present which appear to be elements of an incomplete arch. 

 A small, short vessel arises from the fourth arch between its first 

 and second thirds. Three small sinuses lie in the region just beneath 

 the blind end of this vessel, and extend toward the ventral end of 



27* 



