270 ZOOLOGY. 



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middle line as soon as they can. The adult division 

 into right and left auricles and ventricles is of much later 

 origin, and has nothing to do with this temporary arrange- 

 ment, which is simply a mechanical result of the slitting 

 open of the floor of the pharynx, due to the spreading out 

 of the embryo over the surface of the' blastodermic vesicle. 



The most important vessels at first in the chick are those of the 

 yolk-sac, since they are essential to the nourishment of the embryo. 

 They begin to appear in the area opaca during the first day of 

 incubation. Mesoblast cells elongate and fuse with one another into 

 a kind of network : vacuoles (i.e. cavities containing liquid) are 

 formed in their protoplasm, and increase until they form continuous 

 channels, with the remainder of the protoplasm for their walls. 

 These are the earliest capillaries. The nuclei of the cells that form 

 them divide, and small cells are budded off into the cavities : these 

 are the first blood-corpuscles. The heart and the larger vessels are 

 formed in a different way, from bundles of cells the cavities being 

 formed between the cells, not in their protoplasm. All the early 

 vessels are formed in splanchnic mesoblast : the cardinal veins are 

 the first important ones in somatic mesoblast. 



Taking the chick as the most convenient type, the main 

 course of the circulation at first (during the second day of 

 incubation) is as follows. From the yolk-sac, by the two 

 viteUine veins (whose proximal part represents the split 

 sub-intestinal vein) to the heart', thence by the aortic 

 arches (lying in the branchial arches) to the dorsal aortse 

 and back to the median aorta ; thence by vitelline arteries 

 to the yolk-sac again. 



6. Later Embryonic Circulation. On the third day 

 of incubation we have, in addition to the above, the an- 

 terior and posterior cardinals (fig. 143, A) ; and on the 

 fourth, with the great development of the mesonephros and 

 the allantois, we find the median postcaval and the allantoic 

 veins appearing as important factors of the original sub- 

 intestinal (fig. 143, B). 



While these changes are going on, changes have also 

 been taking place in the aortic arches. In the chick, six 

 pairs of complete arches are formed in all in the mandi- 

 bular, hyoid, and four branchial arches. (In the frog, it 

 will be remembered, the first two of these are incomplete, 



