DEVELOPMENT OF THE CIRCULATORY SYSTEM. 931 



tubes. Soon after the external and the internal blastodermic membranes have become 

 separated from each other, and the intermediate membrane has been formed at the 

 thickened portion of the ovum which is destined to be developed into the embryon, cer- 

 tain of the blastodermic cells undergo a transformation into blood-corpuscles. These are 

 larger than the blood-corpuscles of the adult and are generally nucleated. At about the 

 same time (it may be before or after the appearance of the corpuscles, for this point is 

 undetermined) certain of the blastodermic cells fuse with each other and arrange them- 

 selves so as to form vessels. Leucocytes are probably developed in the same way as the 

 red corpuscles. The vessels thus formed constitute the area vasculosa, which is the 

 beginning of what is known as the first circulation. 



It is evident that the relations of the embryon at different stages of development must 

 require certain variations in the arrangement of the circulatory system. The ovum nas, 

 of course, no vascular connection with the mother before the formation of the allantois, 

 It has undergone, however, a certain degree of development, and presents a circulator^ 

 system, which extends over the umbilical vesicie. This stage of development of the vas- 

 cular system constitutes what is known as the first circulation. As the allantois is devel- 

 oped, the circulation over tne umbilical vesicle becomes unimportant, and its vessels disap- 

 pear. Vessels then extend into the allantois, are finally developed into the foetal portion 

 of the placenta, and what is known as the second circulation is established. This circu- 

 lation continues throughout intra-uterine life, and, as we know, the embryon and foetus 

 depend entirely upon the placenta for materials for respiration, nutrition, and growth. 

 At birth, the requirements are again changed. The placental circulation is then abol- 

 ished, and the arrangement of vessels peculiar to it disappears. Now, for the first 

 time, the pulmonary circulation becomes important. All the blood passes through the 

 lungs before it is sent to the general system, the two sides of the heart become com- 

 pletely separated from each other, and the third, the pulmonary, or adult circulation, is 

 established. 



The First, or Vitelline Circulation. In the development of oviparous animals, the 

 first, or vitelline circulation is very important ; for, by these vessels, the contents of the 

 nutritive yolk are taken up and carried to the embryon, constituting the only source of 

 material for its nutrition and growth. In mammals, however, nutritive matter is ab- 

 sorbed almost exclusively from the mother, by simple endosmosis, before the placental 

 circulation is established, and by the placental vessels, at a later period. The vitelline 

 circulation is therefore not important, and the vessels disappear with the atrophy of the 

 umbilical vesicle. 



The area vasculosa, in mammals, consists of vessels coming from the body of the 

 embryon, forming a nearly circular plexus in the substance of the vitellus, around the 

 embryon. The vessels of this plexus open into a sinus at the border of the area, called 

 the sinus terminalis. It is probable that these vessels are developed de novo in the inter- 

 mediate blastodermic layer and are not preceded by a distinct membrane ; but such a 

 membrane has been described under the name of the vascular blastodermic layer. 



If we examine the ovum when the area vasculosa is first formed, we see the embryon 

 lying in the direction of the diameter of the nearly circular plexus of blood-vessels. The 

 plexus surrounds the embryon, except at the cephalic extremity, where the terminal 

 sinuses of the two sides curve downward toward the head, to empty into the omphalo- 

 mesenteric veins. As the umbilical vesicle is separated from the body of the embryon, 

 it carries the plexus of vessels of the area vasculosa with it, the vessels of communication 

 with the embryon being the omphalo-mesenteric arteries and veins. As these processes 

 are going on, the great central vessel of the embryon becomes enlarged and twisted upon 

 itself, at a point just below the cephalic enlargement of the embryon, between the inferior 

 extremity of the pharynx and the superior cul-de-sac of the intestinal canal. The exca- 

 vation which receives this vessel is called the fovea cardiaca. The different stages of 



