512 



At 11 days, 5,0 mm, the pulmonary arch passes behind the post- 

 branchial bodies and is complete. It enters the dorsal aorta by two 

 branches. From one of these a blind sprout tends to pass around 

 the postbranchial body. Ventrally a slender vessel which cannot be 

 traced with certainty passes over the surface of this body but it does 

 not unite with the dorsal sprout. The fourth pharyngeal pouch has 

 not appeared. 



A rabbit of 12 days, 5,0 mm, shows an irregular plexus around 

 the postbranchial body. The fourth pouch has formed but does not 

 extend to the ectoderm. It is seen that most of the plexus is between 

 the fourth pouch and the postbranchial body, but it does not con- 

 stitute a complete aortic arch on either side of the embryo. 



At I2V2 days, 7,0 mm, the fourth pouch and postbranchial bodies 

 are coming into relation with each other, and the irregular small vessels 

 have largely disappeared. 



A pig embryo of 5,5 mm shows the fourth and pulmonary arches 

 completely formed. There is no fourth pouch. The pulmonary arch 

 passes behind the region of the postbranchial body. As in the rabbit, 

 the dorsal portion of the pulmonary arch presents irregularities. 



In a 6,0 mm embryo, the arrangement of vessels is similar to 

 that figured by Miss Lehmann i). In this instance the irregular vessels 

 near the dorsal aorta which seem to have arisen in connection with 

 the pulmonary arch can hardly be described as a fifth arch, especially 

 since the fourth pharyngeal pouch has not yet appeared. 



A pig of 9,0 mm shows the fourth pouch reaching to the ecto- 

 derm. The pulmonary arch is behind the postbranchial body, and 

 divides before entering the dorsal aorta. No fifth arch is found, 

 although as in the last stage a blind sprout leaves the fourth arch 

 midway in its course. Such a sprout in the rat was identified by 

 Tandler as the beginning of a fifth arch. 



In a 10 mm pig, the postbranchial body and the fourth pouch 

 have approached one another and suggest the Y body of following 

 stages. The fourth pouch is partly hidden behind the fourth arch 

 which passes between it and the ectoderm. Irregular vessels remain 

 associated with the pulmonary and aortic arches, but these soon 

 disappear. At 12 mm not more than traces of them are found. 



Summary. 

 The difficulties presented by the posterior portion of the mam- 

 malian branchial system are chiefly those of morphological inter- 



]) H, Lehmann, Anat, Anz., Bd, 26, 1905, Tig, 10, p. 418. 



