EARLY DEVELOPMENT OF THE HEART AND PAIRED BLOOD-VESSELS 255 



cells, the cytoplasm of which shows a darkly staining granular endoplasm and a 

 clear hyaline exoplasm (Fig. 246). According to Wright (Jour. Morphol., 

 vol. 21, pp. 265-278), the blood plates arise by being pinched off from cytoplasmic 

 processes of the giant cells. Wright has shown that genuine blood plates and 

 giant cells occur only in mammals. 



The granules of the plates are interpreted by Wright as derived from the endoplasm of 

 the giant cells and stain in a similar manner. This view has been generally accepted by Ameri- 

 can embryologists who have seen Wright's preparations. Schafer regards the blood plates as 

 minute cells, and the granular endoplasm of Wright as a small nucleus. 



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 probably 

 from the splanchnic mesoderm. The first vessels derived from the angioblast 

 (see p. 251) are small isolated blood spaces which unite and form capillary net- 

 works. From these endothelial sprouts grow out, meet and unite until complete 

 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. The origin of the heart and 

 paired vascular trunks of human embryos is in doubt, but some facts are certain 

 from our study of their development in birds and mammals. 



According to His and Minot, all the blood-vessels of the embryonic body arise as endo- 

 thelial ingrowths from the primitive vascular area of the yolk-sac. According to the investi- 

 gations of Mollier (Hertwig's Handb., vol. i, 1906), in all vertebrates the endothelial anlage 

 of the heart is represented by cells which appear independent of the vascular area between the 

 entoderm and the mesoderm in the distal portion of the head. These vascular cells occur 

 as paired anlages. According to Mollier, vascular anlages arise in situ and give rise to the endo- 

 thelium of the heart. Similarly other vascular anlages form the primitive aortic trunks. 



Evans (Amer. Jour. Anat., vol. ix, 1909) by injecting young chick em- 

 bryos has shown conclusively that most of the descending aorta "is formed from 

 the medial margin of the vitelline capillary plexus." In mammals, connection 

 of the same plexus with the descending aorta has been demonstrated by Turs- 

 ting. Bremer (Amer. Jour. Anat., vol. 13, 1912, pp. 111-128) in summarizing 

 his work on the development of the aorta and aortic arches in the rabbit says: 

 "The dorsal aorta, the first aortic arch, the conus arteriosus, and the lateral heart 

 are all parts of an original network of angioblast cords derived from the extra- 

 embryonic plexus of blood-vessels." 



Bremer points out that as the true vessels with cavities develop they are connected by 

 intervening cords of the angioblast. This connection cannot be demonstrated by injection 



