THE HEART AND BLOOD-VESSELS. 165 



I) KVELOPMENT OF THE HEART AND BLOOD- 

 VESSELS. 



1 . Preliminary Account. 



The blood-vessels arise in the mesoblast. Id most parts of 

 the body of the tadpole they appear first as irregular spaces or 

 lacunas, formed by separation of the mesoblast cells from one 

 another. These lacunar spaces are at first independent, but 

 soon extend so as to meet and open into one another as irregular 

 channels. The cells surrounding these channels assume more 

 definite arrangement and character, and in this way the chan- 

 nels become converted into blood-vessels. The blood corpuscles 

 are either cells which are inclosed from the first within the 

 lacunar spaces, or more usually are cells budded off at a later 

 stage from the walls of the vessels into their cavities. 



Each blood corpuscle is a single cell. In the earl}' stages of 

 development all the blood corpuscles of the frog embryo are alike, 

 consisting of spherical cells in which are imbedded numerous yolk- 

 granules. These yolk-granules are gradually used up for the 

 nutrition of the embryo, and shortly after the hatching of the 

 tadpole the corpuscles begin to acquire the shape and characters 

 of the red blood corpuscles in the adult frog. 



The chief point of interest in the development of the blood- 

 vessels of the frog is afforded by the changes which occur during 

 the transition from the gill-breathing to the lung-breathing condi- 

 tion. 



While the tadpole is breathing by means of gills its circula- 

 tion is in all essential respects that of a fish. The venous blood, 

 returned from the body at large, enters the posterior end of the 

 heart, or sinus venosus : from this it passes into the second or 

 auricular chamber, thence to the ventricle, and from that to 

 the truncus arteriosus (Fig. 64). The blood passes through the 

 several cavities in succession, there being as yet no division 

 between the right and left sides of the heart. 



The truncus arteriosus divides distally into right and left 

 branches, from each of which four afferent branchial vessels (Fig. 

 76, af) arise. The four vessels of each side run outwards along 

 the hinder borders of the four branchial arches, giving off' 

 along their whole length numerous branches to the gill-tufts 



