THE DEVELOPMENT ‘OF THE EMBRYO ITSELF. 97 
The entire blood-vascular system originates in the mesoblast 
of the area opaca especially ; at first appearing in isolated spots 
which come together as actual vessels are formed. The student 
who will pursue the plan of examining a series of incubating 
eggs will be struck with the early rise and rapid progress of the 
Fie. 109.—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 vesicles ; OP, optic 
vesicle ; ot, auditory vesicle ; ofv, omphalo-mesenteric veins ; Ht, heart ; Ao, bulbus arte- 
riosus ; ch, notochord ; Ofa, omphalo-mesenteric arteries ; Pv, protovertebree ; x, point of 
divergence of the splanchnopleural folds ; y, termination of the foregut, v. * 
vascular system of the embryo, which takes, when complete, 
such a form as is represented diagramatically in Fig. 113. 
The blood and the blood-vessels arise simultaneously from 
the cells of the mesoblast by outgrowths of nuclear prolifera- 
tion, and in the case of vessels (Fig. 147) extension of processes, 
fusion, and excavation. 
The fore-gut is formed by the union of the folds of the 
splanchnopleure from before backward, and the hind-gut in a 
similar manner by fusion from behind forward. 
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