Embryonic history of the aortic arches iu mammals. 415 



The left aortic root, below its union with the sixth arch, is 

 slightly larg-er than the right. 



Dorsal remnants of the first two arches still persist, and also 

 a small ventral remnant of the second arch is present, as shown in 

 Fig. 18 X, Plate 24. This corresponds in position to the similar 

 rudiment shown in Fig. 16, Plate 24. 



The dorsal ends of the fourth and sixth arches are apparently 

 in an unstable condition and may be compared with the correspond- 

 ing vessels in the four following stages. 



Embryo of the twentieth day (No. 4, somewhat older). 

 This embryo is also of the twentieth day. Its external appearance 

 is shown in Fig. 19, Plate 22. There are four well developed 

 branchial pouches. A well developed maxillary process of the man- 

 dibular pouch is present, as in corresponding stages of the rabbit. 



In Fig. 20, Plate 24, we have a reconstruction of the left side 

 of the same embryo showing three complete aortic arches, and 

 rudiments of three incomplete ones. 



A very small dorsal remnant of the first arch is present (Fig. 20, 1, 

 Plate 24); it is directed upward and outward from the aortic root. 

 A short distance anterior to the truncus arteriosus lies a sinus which 

 extends into the mandibular visceral arch, and appears to be very 

 slightly connected with a distinct but slender vessel which leaves 

 the ventral end of the third arch. This vessel joins the anterior 

 wall of the third arch at a decidedly greater distance from the 

 median line than the similar vessel in the preceding embryo. Upon 

 the right side an extremely small branch leaves this vessel a short 

 distance above its union with the third arch, and forms a slight 

 connection with the anterior prolongation of the truncus, near the 

 median plane. I am unable to find a similar structure on the left side. 



A larger dorsal remnant of the hyoid aortic arch is present, 

 passing outward and then downward for a short distance, through 

 the corresponding visceral arch. The ti-uncus arteriosus projects 

 upward in front of the ventral end of the third arch, in the form 

 of a pocket as has been described for earlier stages. The third 

 arch is smaller than in the preceding embryos, and a noticable 

 decrease has taken place in the size of the dorsal aorta between 

 the third and fourth arches. The fourth arch is the largest of the 

 complete arches, and certain peculiarities in its structure are to be 

 noted. A large posteriorly directed spur leaves this vessel a little 

 ventral to its middle region. At the dorsal end of the fourth arch, 



