DEVELOPMENT OF THE HEART 247 



' 



Origin of the Blood Plates. In the bone marrow and spleen pulp are 

 giant cells, or megakaryocytes, the cytoplasm of which shows a darkly 

 staining granular endoplasm and a clear hyaline ectoplasm (Fig. 253). 

 It has been shown by Wright (1910) and others that the blood plates arise 

 by being pinched off from cytoplasmic processes of the giant cells. The 

 central granular mass of the plates represent portions of the endoplasm. 

 Genuine blood plates and giant cells occur only in mammals. 



DEVELOPMENT OF THE HEART 



Vasculogenesis . We have seen that the first blood cells and blood vessels 

 take their origin in the angioblast, which develops in the wall of the yolk sac 

 and chorion from the splanchnic 

 mesoderm. The first vessels 

 derived from the angioblast 

 (see p. 243) are small, isolated 

 blood spaces which unite and 

 form capillary networks. From 

 these, endothelial sprouts grow 

 out, meet, and unite until com- 

 plete networks are formed. In 

 human embryos of i mm. or less, 

 these envelop the lower portion 

 of the yolk sac, the body stalk, 

 and chorion. 



There are two views as to 

 the manner in which the heart 

 and the primitive vascular 

 trunks of the embryo originate. 

 According to His and Rabl, and 

 more recently Minot, Evans, 

 and Bremer, all the blood vessels 

 of the embryonic body arise as 

 endothelial ingrowths from the 



FIG. 254. The caudal end of a chick embryo 

 of 32 somites, showing the primary capillary plexus 

 in the posterior lirnb buds from which the sciatic 

 artery will differentiate. Aortae have formed 

 from the mesial margins of the plexuses (Evans). 



extra-embryonic yolk-sac angio- 

 blast. Kolliker, Rucket, and 

 Mollier (1906), on the contrary, 

 assert that the intra-embryonic 



vessels are formed by the fusion of discrete anlages in a way similar to 

 that first occurring on the yolk sac. Corroborative investigations by 

 Maximow, Huntington, Schulte, and others have shown that the apparent 

 invasion of angioblast in reality represents a progressive fusion of isolated 

 mesenchymal tissue spaces. Moreover, direct experimental proof on 



