1188 



EMBRYOLOGY. 



the blood is returned by two vessels (veins) which enter the heart posteriorly, and 

 thus a complete circulation is formed (Fig. 744). 



In these vessels and the heart a fluid (blood) is contained, in which rudi- 

 mentary corpuscles are found. The mode of formation of these elementary parts 

 must, first be considered. 



In mammalia the heart is formed by a longitudinal fold of the splanchnopleure 

 with its underlying hypoblast on either side of the median line in front of the 

 anterior extremity of the rudimentary pharynx, at about the level of the posterior 

 primary cerebral vesicle. The folds become tubes, their walls thicken and present 

 two distinct strata of cells : the inner and thinner layer, derived from the hypo- 

 blast, forms the endocardium ; the outer and thicker, derived from the visceral 

 mesoblast, forms the muscular wall of the heart. In its primitive condition, the 

 heart consists therefore of a pair of tubes, one on either side of the body. These, 

 however, soon coalesce in the median line, and, fusing together, form a single 

 central tube. 1 Each of the two primary tubes receives posteriorly a large vein 

 (the omphalo-mesenteric vein), and is prolonged anteriorly into an artery (the 

 primitive aorta). So that after fusion of the heart-tubes has taken place there is, 

 in the primitive vitelline circulation, as above mentioned, a single tubular heart, 



Descending aorta. 



Primitive jugular, 

 vein. 



Amnion. 



Cardinal vein. "/SJ 



Descending aorta. 



Abdominal stalk_ 

 (lianchstiel). 



Chorionic mill. 



FIG. 744. Human embryo of about fourteen days, with yolk-sac. (After His.) (From Kollmann's 



Entwickelungsgeschichte. ) 



with two arteries proceeding from it and two veins emptying themselves into it. 

 The first blood-vessels are developed as follows : The nucleated embryonic cells of 

 the mesoblast send out processes in various directions. These processes fuse to- 

 gether, and an irregular network is formed. The nuclei of the cells multiply, and, 

 accumulating around themselves a small quantity of the protoplasm of the cell, 

 they acquire a tinge of color and form the first red blood-corpuscles. The proto- 

 plasm of the cells and their branched network become hollowed out into a system 

 of canals containing fluid, in which the newly formed corpuscles float (Fig. 745). 2 



The earliest blood-corpuscles are all nucleated, and in this and other respects 

 that is, in their possession of amosboid movements and in their capability of under- 



1 In most fishes and in amphibia the heart originates as a single median tube. 



5 Recent observers incline to the view that the blood-corpuscles are of hypoblastic origin, being 

 developed from the endothelium of the vessels, the sequence of the development of the different 

 structures being : first the heart, then the blood-vessels, and lastly the blood-corpuscles. (Consult 

 Dr. Ernest Mehnert's iomeckani/e, Jena, 1898.) 



