Lilian V, Sampson 



493 



cf the visceral pouches, that the observations may be amplifiecl by the 

 study of more abundant material. The vessels of the arches are charac- 

 terized by extreme simplicity. The first vessel to be completed is the 

 aortic arch of the first branchial arch in stage l\ ; very soon the vessel of 

 the second arch is also complete. In the meantime incomplete vessels or 

 lacunfe are found in the mandibular and hyoid arches, but at no time is 

 there a complete vessel from the truncus to the collecting aorta in any 

 visceral arch save in the first and the second branchial. 



The pulmo-cutaneous vessel when first seen in the series, at about stage 

 V to VI, branches near the truncus from the aorta of the second branchial 

 arch. A study of fairly abundant material shows no indication of the 

 origin of the pulmo-cutaneous from an artery of the fourth branchial arch, 

 as in the frog. There is no representative of the vessel of the third bran- 

 chial arch. 



The condition then in stage VI (Fig. P) is as follows :— The first bran- 

 chial arch passes from the truncus into the .collecting aorta, and is con- 

 tinued anteriorly as the carotid. The aorta of the second branchial arch 

 passes posteriorly as the systemic, meeting the corresponding vessel of the 

 other side, posterior to where each has supplied the glomus of the head- 

 kidney. The collecting aorta is ca 

 already somewhat reduced between 

 the dorsal parts of the first two 

 arches. Lacuna are still found in 

 the mandibular and hyoid arches ; 

 the vessel in the hyoid is con- 

 nected with the ventral portion of 

 the aorta of the first branchial, 

 but the aorta is not complete in 

 either mandibular or hyoid arch. 

 Finally, at the point where the 

 first and second branchial arter- 

 ies separate, a more posterior 

 branch, the pulmo-cutaneous, is given off. 



In stages soon after VI. as the visceral arches are obscured, the collect- 

 ing aorta between the carotid and systemic degenerates, and the embryonic 

 ai-rangement of the aortic arches cannot be detected ; the carotid, systemic 

 and pulmo-cutaneous arise as three great branches from the truncus. 



Pulmonary circulation. — The study of the pumonary vessels has given 

 mostly negative results. The cutaneous artery is an established vessel 

 from stage VI on, but in the same section where it can be clearly traced, 

 nothing can be seen of the pulmonary artery in specimens younger than 



FiQ . p. Sagittal section of the head in stage 

 VI. aoi, ao.,, aorta of the 1st and of the 2d 

 branchial arch : be, body cavity ; ca, carotid 

 artery; da, sj'stemic aorta: hk. head-kidney; 

 pcv, posterior cardinal vein; y, yolk-mass. 



