IV.] 



THE BLOOD-VESSELS. 



67 



distributed over the pellucid and vascular areas, the main 

 trunk of each aorta passing on with greatly diminished calibre 

 towards the tail, in which it becomes lost. 



Fig. 17. A. 



Fig. 17. B. 



Two consecutive Sections op a 36 hours' Embryo illustrating the 



FORMATION OF THE HEART. A IS THE MOST ANTERIOR OF THE TWO. 



h. b. hind brain. nc. notochord. E. epiblast. so. somatopleure. sp. 

 splanchnopleure. d. alimentary canal, hy. hypoblast, hz. (in A) heart. 

 of. omphalo-mesaraic vein. 



The heart is seen from the sections to be formed from the mesoblast of the 

 splanchnopleure. It is not however split off from a portion of the mesoblast 

 which forms the muscular wall of the alimentary canal, but the mesoblast, where 

 it turns round to run outwards again over the yolk-sack, becomes thickened, 

 and in each of the thickenings (one on each side) so formed, a cavity appears 



5—2 



