419 



arches, the third, fourth and sixth, and rudiments of three incomplete 

 ones, viz., the first, second and fifth. 



A very small dorsal remnant of the first arch is present. There 

 is a relatively large dorsal remnant of the second arch. The truncus 

 arteriosus projects upward in front of the third arch in the form of 

 a pocket. 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. 



Between the fourth and sixth arches are rudiments of the fifth 

 aortic arch consisting of, a spur from the fourth arch, and three from 

 the dorsal aorta and sixth arch, with a chain of sinuses between them. 

 In this specimen two distinct entodermal pouches are present between 

 the fourth and sixth arches. 



The pulmonary aortic arch in this stage has manifestly increased 

 in size so that its diameter nearly equals that of the third arch. The 

 rudiments of pulmonary arteries are developed, symmetrically as to 

 position, one from the posterior side of either sixth arch, about midway 

 in its course. The pulmonary artery of the left side is, however, 

 a more slender vessel than that of the right. 



Embryo near the close of the twentieth day. The 

 reconstruction of this embryo, Fig. 11, shows small dorsal rudiments 

 of the first and second arches, three complete arches, and rudiments 

 of the fifth. These may be compared with the fifth arch shown in 

 Fig. 12. The forward extension of the truncus arteriosus mentioned 

 in earlier stages (not represented in rabbit embryos) is for the first 

 time absent in this stage of the pig. 



Embryo of the twenty -first day. Fig. 12 shows a re- 

 construction of the aortic arches of the left side of this embryo. Short 

 spurs on the aortic roots are present in the mandibular and hyoidean 

 visceral arches, which correspond in position to the dorsal remnants 

 of the first and second aortic arches in younger embryos. 



A long, slender vessel runs forward from the ventral end of the 

 third arch of either side, leaving this artery slightly above the union 

 of the third pair of arches. Its course lies for the most part parallel 

 to the anterior prolongation of the dorsal aorta, and just back of the 

 cleft between the mandibular and hyoidean visceral arches, it divides 

 into three small branches. The course of these vessels upon either 

 side is symmetrical except near their union with the third pair of 

 arches; here the vessel from the left side approaches the median 

 line while the one from the right joins the third arch of that side at 

 some distance from the median plane, its point of connection cor- 



27* 



