102 



COMPARATIVE PHYSIOLOGY. 



hollow prior to their coalescence, when the two tubes become 

 one. 



The entire blood- vascular system originates in the mesoblast 

 of the area opaca especially ; at first appearing in isolated spots 



Fio. lOS.— Chick on third day, seen from beneath as a transparent object, the head 

 being turned to one side {Foster and Balfour), a', false amnion; a, amnion; CH, 

 cerebral hemisphere; FB, MB, HB, anterior, middle, and posterior cerebral vesi- 

 cles; OP, optic vesicle; o*, auditory vesicle; Q^F, omphalo-mesenteric veins; Bt, 

 heart; Ao, bulbus arteriosus; eh, notochord; Of.a, omphalo-mesenteric arteries; 

 Pv, protovertebrse; x, point of divergence of the splanchnopleural folds; y, ter- 

 mination of the fore-gut, F. 



which come together as actual vessels are formed. The student 

 who wiU. pursue the plan of examining a series of incubating 

 eggs will be struck with the early rise and rapid progress of the 

 vascular system of the embryo, which takes, when complete, 

 such a form as is represented diagramatically in Fig. 109. 



The blood and the blood-vessels arise simultaneously from 



