DF.VELOPMENT OF RANA 187 



into the head, the internal carotids. Behind, they join and 

 fuse together along the whole of the rest of the body, forming 

 the single dorsal aorta. 



Beneath the pharynx, the heart communicates forwards 

 with the ventral aorta. In each of the 3rd to 6th visceral 

 arches, between the gill-slits, a vessel appears which com- 

 municates below with the ventral aorta and above with the 

 lateral dorsal aorta of its own side. In this way the series of 

 pairs of aortic arches arise, alternating with the gill-slits. When 

 the capillaries of the gills arise, they connect with the aortic 

 arches which become interrupted. There are now afferent 

 branchial arteries carrying blood from the ventral aorta to the 

 gills, and efferent branchial arteries connecting the gills with 

 the lateral dorsal aorta. Rudiments of aortic arches appear 

 in the mandibular and hyoid arches. 



The dorsal aorta sends arteries to the gut, which they reach 

 by passing down between the two layers which form the dorsal 

 mesentery. 



The arteries become surrounded by coats of smooth muscle. 

 Of the veins, the posterior cardinals arise near and parallel 

 to the dorsal aorta. Their anterior prolongations are the 

 anterior cardinal veins which run one on each side of the brain, 

 and which, later on, contribute to the formation of the internal 

 jugulars. At this period, the pericardial cavity is open 

 posteriorly and communicates with the general perivisceral 

 splanchnoccel. In the region of the heart, the splanchnic 

 and somatic layers of the ccelomic epithelium approach one 

 another and fuse, forming the lateral mesocardia which connect 

 the gut- wall with the body- wall. This connexion of course 

 interrupts the coelomic cavity, and soon the pericardial cavity 

 is completely shut off from the perivisceral cavity behind it. 

 The partition formed by the lateral mesocardia is the transverse 

 septum, and it is important in that it enables the cardinal veins, 

 which are in the body-wall, to communicate via the ductus 

 Cuvieri (or superior venae cava?) with the heart, which is of 

 course situated in the gut- wall. 



A third connexion between the heart and the veins of the 

 body-wall is established by the formation of the inferior vena 



