DEVELOPEMENT OF EEPTILIA. 631 



but a temporary air-breathing organ is substituted to remove the 

 carbon as the organic machine becomes more complex, and its 

 actions more vigorous and various. From the fore-part of the 

 cloaca a vesicle is protruded which elongates, escapes by the um- 

 bilicus, and, carrying along with it blood-vessels, applies their 

 ramifications to the inner side of the shell : this is the ( allantois,' 

 figs. 445 and 450, b. 



On the part of the yolk supporting the embryo blood-channels 

 appear which form a circular canal called ' vena termiualis ; ' it 

 bends towards the embryo at the part near the head, and passes 

 through the opening of the cephalic hood to a transverse canal, 

 6 vena aiferens,' behind the heart: this is now an obliquely bent tube, 

 which pulsates and sends the circulating fluid to a dorsal vessel, 

 which soon distributes vessels, ris;ht and left, in the abdominal 



J O 



region to the ( vena termiualis.' towards which numerous chan- 



~ * 



nels pass from the included space, fig. 450, c, the whole now 

 forming the ( area vasculosa ' upon the yolk. The fluid first 

 circulated in this system of channels is pale plasma with granules. 1 

 The first circulation in an amniotic embryo may be described as 

 passing from the heart-tube by vascular arches to the ' dorsal 

 artery,' which supplies the parts of the embryo, and sends ( om- 

 phalo-meseraic ' branches to the e area vasculosa,' from the f vena 

 terminalis ' of which area the blood returns by the ( vena afFerens ' 

 to the heart. The dorsal artery bifurcates posteriorly, and returns 

 along; the abdomen as the e veua3 cardinales : ' the arteries to the 



o 



head also return as ' precaval veins,' and all these terminate in 

 the f vena afFerens.' Dilatations of the heart-tube indicate a 

 ventricle, fig. 443, a, and a e bulbus arteriosus, ib. b : ' the latter is 

 more prominent at first. An auricular dilatation behind the ven- 

 tricle next appears. A protuberance in advance of the caudal 

 curvature is formed by what soon is recognisable as a hollow sac, 

 ib. d\ which, as it expands, carries with it branches from the 

 dorsal artery : these are the f umbilical ' or ( allantoic ' arteries, 

 fig. 450, i, which convey, as the bag protrudes and expands, part 

 of the circulation to receive the influence of the air through the 

 pores of the shell ; and, the blood returning by the ' umbilical ' or 

 ' allantoic veins,' a subsidiary circulation to the vitelline, ib. c, is 

 established, analogous to the branchial one of Batrachians and 

 Fishes. The blood has now become red, and of shades indicating 

 its arterial and venous conditions. The blood-corpuscles, at first 

 globular, become slightly flattened, but the discs are circular be- 

 fore acquiring their elliptical form. 2 The omphalo-mesenteric 



1 Hunter, CCLXXX. (1794) p. 45, and xx. vol. v. p. xxiv. 



2 When the heart begins to lose its tubular shape the blood-particles are minute 



