758 



THE VASCULAR SYSTEMS 



DEVELOPMENT OF THE BLOOD- VASCULAR SYSTEM. 



There are three distinct stages in the development of the circulatory system, each in accordance 

 with the manner in which nourishment is provided for at different periods of the existence of 

 the individual. In the first stage there is the vitelline circulation, during .which nutriment is 

 extracted from the vitellus or contents of the yolk sac. In the second stage there is the placenta} 

 circulation, during which nutriment is obtained by means of the placenta from the blood of the 

 mother. In the third stage, commencing after birth, there is the complete circulation of the 

 adult, during which nutrition is provided for by the organs of the individual. 1 



Ectoderm. 



Mesoderm. 

 Entoderm. 



Blood island. 



FIG. 530. Section through vascular area to show commencing development of bloodvessel. 



(Semidiagrammatic.) 



Bloodvessels first make their appearance in the mesodermal wall of the yolk sac, i. e., outside 

 the body of the embryo. Here the cells become arranged into solid strands or cords which join 

 to form a close-meshed network. The peripheral cells of these strands become flattened and 

 joined to each other by their edges to form the walls of the primitive bloodvessels. Fluid col- 

 lects within the strands and converts them into tubes, and the more centrally situated cells of 

 the cell cords are thus pushed to the sides of the vessels and appear as masses of loosely arranged 

 cells which project toward the lumen of the tube. These masses are termed blood islands (Fig. 

 530) ; their cells acquire coloring matter (hemoglobin), and are then detached to form the blood- 

 corpuscles or erythroblasts (Fig. 531). 2 The earliest blood corpuscles are all nucleated; they 



Ectoderm. 



Vessel wall. Blood-corpuscles. 

 FIG. 531. Later stage. 



are also capable of subdivision and of executing ameboid movements, and in these respects 

 resemble colorless blood corpuscles. Soon, however, true colorless blood corpuscles make their 

 appearance, and, according to Beard, 3 are first derived from the rudiments of the thymus. 



Coincidently with the development of the bloodvessels in the vascular area, the first rudi- 

 ment of the heart appears as a pair of tubular vessels which are developed in the splanchno- 

 pleure of the pericardial area. These are named the primitive aortce, and a direct continuity is 

 soon established between them and the vessels of the vascular area. Each receives anteriorly 

 a vein the vitelline vein from the yolk sac, and is prolonged backward on the lateral aspect of 

 the notochord under the name of the dorsal aorta. The dorsal aortse end at first on the yolk sac; 

 but with the development of the allantois, they are continued onward through the body stalk 

 as the umbilical arteries to the villi of the chorion. 



By the forward growth and flexure of the head the pericardial area and the anterior portions 

 of the primitive aortse are folded caudad on the ventral aspect of the fore-gut, and the original 



1 That the umbilical circulation precedes the vitelline in the human embryo seems to be shown by Eternod's 

 and Dandy's independent observations. Consult the latter's article, A Human Embryo with Seven Pairs of 

 Somites, American Journal of Anatomy, January, 1910. 



2 According to Dandy (loc. cit.) there is at first no apparent connection between this blood-forming area and 

 the vascular system of the very young human embryo, and that the presence of blood corpuscles in the latter 

 is probably explained by endothelial proliferation from the capillaries in the chorionic membrane. 



* Anatomischer Anzeiger, December, 1900. 



