DEVELOPMENT OP CONNECTIVE SUBSTANCE AND BLOOD. 177 



make it arise either out of the middle germ-layer alone, or out of 

 the entoblast alone, or by the migration of cells out of both layers 

 and their union into a single fundament. StUl other variations 

 result from the first fundament of the blood-course being some- 

 times referred to a limited territory of the germ, sometimes to several 

 places. Thus, for the meroblastic eggs of Birds, the area opaca is 

 designated by some observers as the place where vessels and blood 

 are first formed. From here they grow out as it were at first into the 

 embryonic body proper. The opposite is reported of Bony Kshes, in 

 which the first vessels, heart, aorta, caudal veins, and sub-intestinal 

 veins, together with blood-corpuscles, arise earliest in the embryonic 

 body itself, whereas they appear on the yolk only subsequently. 

 Finally, for the Selachians a local origin of the vessels is maintained 

 both for the area opaca and also for the embryonic body in the 

 restricted sense. 



In opposition to the two views hitherto presented, a third view 

 assumes a separate origin for the connective substances on the one 

 hand, and for the vascular endothehum and the blood on the other. 

 Whereas the former are produced by the emigration of cells from the 

 middle germ-layer, the vascular endothelium is maintained to arise 

 from cells of the entoblast. It is held that an endothelial sac is 

 formed (perhaps by constriction) as an independent fundament, 

 which by budding gives rise to the whole vascular system. 



After this brief survey of the various possibilities concerning the 

 origin of the blood-course, I turn to a description of certain con- 

 ditions, concerning the signification of which it must be admitted 

 that the views are also often very divergent. 



The area opaca of the meroblastic eggs of Fishes, Reptiles, and 

 Birds has always played an important r61e in the literature on the 

 question of the origin of the blood. Notwithstanding the frequency 

 with which it has been investigated, the researches concerning it 

 cannot be regarded as concluded. It is from this standpoint that I 

 beg the reader to judge what follows. 



In the case of the Chick, on which especially we shall base our 

 account, the opaque area is composed of only the two primary germ- 

 layers at the time when the middle germ-layer begins to be formed 

 from the region of the blastopore by the production of folds. 



The outer germ-layer, as has already been described in Chapter V., 

 has in general a simple structure, since it is composed of a single 

 layer of small cubical cells. The inner germ-layer (fig. 56 ik and 

 fig. 112), on the contrary, alters its condition the more we approach 



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