EARLY DEVELOPMENT OF THE HEART AND PAIRED BLOOD VESSELS 247 



It is commonly held that the eosinophilic granules differentiate endogenously (Downey, 

 Anat. Rec, vol. 8, 1914). However, Weidenreich (Arch. f. mikr. Anat., Bd. 82, 1913) re- 

 gards these granules as ingested fragments of red corpuscles or their hemoglobin derivatives. 

 Badertscher (Amer. Jour. Anat., vol. 15, 1913) found numerous eosinophiles and free eosino- 

 phihc granules in the vicinity of degenerating muscle fibers in salamanders. Also during 

 trichiniasis in man, when there is extensive degeneration of muscle fibers, the number of 

 eosinophiles in the blood becomes greatly increased. 



3. Basophiles, or Mast Leucocytes (Maximow), form only 0.5 per cent, of the 

 leucocytes. Their nuclei are very irregular in form and may be broken down into 

 several pieces which stain intensely. The granules are variable in number, size, 

 and form, and often stain so heavily as to obscure the nucleus. The cytoplasm 

 is clear and vacuolated. Basophiles have been regarded as degenerating granular 

 leucocytes, but at present this view is not generally accepted. 



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 hyahne ectoplasm (Fig. 253) . It has been shown by Wright 

 (Jour. Morphol., vol. 21, 1910) that the blood plates arise by being pinched off 

 from cytoplasmic processes of the giant cells. The granules of the plates are inter- 

 preted as portions of the endoplasm of the giant cells, for they stain in a similar 

 manner. Genuine blood plates and giant cells occur only in mammals. 



EARLY DEVELOPMENT OF THE HEART AND PAIRED BLOOD VESSELS 



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, endotheUal sprouts grow out, meet, and unite until complete net- 

 works are formed. In human embryos of 1 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 endothehal ingrowths from the extra-embryonic yolk sac angioblast. 

 Kolliker, Rlickert, and Mollier (Hertwig's Handb., 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 mesen- 



