DEVELOPMENT OF THE CIRCULATORY SYSTEM. 429 



the first vessels differs from that of vessels formed later, 

 as they appear de novo in the blastodermic layers, while 

 afterward, vessels are formed as prolongations of preexisting 

 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, certain of the blastodermic cells undergo a trans- 

 formation into blood-corpuscles. These are larger than the 

 blood-corpuscles of the adult, and are generally nucleated. 1 

 At about the same time, it may be before or after the ap- 

 pearance of the corpuscles, for this point is undetermined, 

 certain of the blastodermic cells fuse with each other and 

 arrange themselves 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 differ- 

 ent stages of development must require certain variations in 

 the arrangement of the circulatory system. The ovum has, 

 of course, no vascular connection with the mother before the 

 formation of the allantois ; it has undergone, however, a cer- 

 tain degree of development, and presents a circulatory sys- 

 tem, which extends over the umbilical vesicle. This stage 

 of development of the vascular system constitutes what is 

 known as the first circulation. As the allantois is devel- 

 oped, the circulation over the umbilical vesicle becomes un- 

 important, and its vessels disappear. Yessels then extend 

 into the allantois, are finally developed into the foetal por- 

 tion of the placenta, and what is known as the second circu- 

 lation is established. This circulation continues throughout 

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

 depend entirely upon the placenta for materials for respira- 

 tion, nutrition, and growth. At birth, the requirements are 



1 For the differences between the blood-corpuscles of the embryon and the 

 adult, see vol. i., Circulation, p. 119. 



