194 EMBRYOLOGY. 



What is the vascular area (area vasculosa) of the foetus? 



Among the earliest changes in the blastoderm', occurring in the 

 second week, of impregnation, is the formation of blood-vessels 

 and blood-corpuscles. This occurs by the proliferation of certain 

 branched cells of the mesoblast. and these cells form a closed sys- 

 tem of branching capillaries, their nuclei acquiring a red color 

 and becoming the blood-corpuscles. This area is external to but 

 connected with the embryo. 



How is the heart formed? 



At about this time certain cells of the visceral layer of the meso- 

 blast (splanchnopleure) develop a tube upon each side of the body, 

 and these two tubes soon coalesce to form a single tube (Fig. 65), 

 which receives two veins at its lower end and gives off two arteries 

 at its upper. This is the primitive heart, and pulsations begin in it 



very feebly almost as soon as there is 

 FIG. 65. FIG. 66. a trace of the originating cells. This 



2 .,2 structure soon develops a muscular tis- 



sue, and a circulating fluid which short- 

 ly presents the character of blood. 



What is the next important change 

 in the form of the heart ? 



/ ' ' \ ' J It bends itself so as to assume a U- 



FIG 65. Earliest Form of the Fee- shape, which shortly is twisted in such 



StrtSJ : eltS^ eXtreinit7S manner that the arterial end of the 

 FIG. 60. Foetal Heart bent upon heart crosses in front of the venous 

 1 2 ' (Fig. 66), and the loop suggests the 

 outline of the ventricles. The vitel- 

 line circulation in the human ovum is not very long-lived, for the 

 chorion is early formed and the stock of nutrient protoplasm in the 

 yelk sac is very small. 



What further modifications does the heart undergo ? 



The septum between the ventricles grows, and separates the 

 heart into two divisions; and at about the same time the auricles 

 are developed and the valves become well marked. These changes 

 occur in the fourth to the eighth week of embryonic life. 



Describe the vitelline circulation. 



The area vasculosa extends all about the blastoderm upon the 



