590 RESPIRATORY AND CIRCULATORY ORGANS CHAP. 



ectoderm, the latter becoming perforated to form the 

 external branchial apertures. Gill-clefts appear in the em- 

 bryo of reptiles, birds, and mammals animals in which gills 

 are never developed (Fig. 166); but they early disappear 

 with the exception of the first cleft, corresponding with the 

 spiracle of the dogfish, which gives rise in all Vertebrates 

 above fishes to the tympano-eustachian passage (p. 449) : 

 the branchial skeleton, as we have seen, undergoes a cor- 

 responding reduction or modification (pp. 438 and 496). 

 In pulmonate Vertebrates the lungs arise as a ventral 

 outgrowth of the pharynx. 



The circulatory organs are developed from the meso- 

 derm, the heart arising in the visceral layer on the ventral 

 side of the pharynx. It has at first the form of a straight 

 tube, which soon becomes twisted into an S-shape and in 

 which transverse constrictions are formed dividing it into the 

 different chambers. The auricular and ventricular portions 

 are each at first single, but from the Amphibia onwards the 

 former subsequently becomes divided into two by a septum, 

 and the ventricle is similarly subdivided in birds and mam- 

 mals. The modification of the arterial arches in the ex- 

 amples studied has already been described (pp. 452 and 423). 



In the meroblastic eggs of the dogfish and bird the 

 dorsal aorta, in addition to its other branches, gives rise to 

 paired vitelline arteries : these vessels branch up over the extra- 

 embryonic part of the blastoderm (p. 596) which spreads 

 over the yolk, and take an important share in the absorp- 

 tion of the latter by the embryo. From this area vasculosa 

 (Fig. 165) the blood is returned by splanchnopleuric 

 vitelline veins (Fig. 158) which join with a subintestinal 

 vein (compare Amphioxus, Fig. no) and open into 

 vessels which eventually give rise to the hepatic portal 



