840 



EMBRYOLOGY 



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

 the embryo, certain of the blastodermic cells undergo a transformation 

 into blood-corpuscles. These are larger than the blood-corpuscles of 

 the adult and usually are nucleated. At about the same time certain of 



Fig. 243. Area vasculosa of a rabbit of ten days (van Beneden and Julin) . 



In this figure the arteries, the arterial capillaries and the sinus terminalis (arterial) are in red, and 

 the veins and venous capillaries are in blue. 



the blastodermic cells fuse with each other and arrange themselves so 

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

 as the red corpuscles. The vessels thus formed constitute the area vas- 

 culosa, which' is the beginning of what is known as the first circulation. 

 The cells of the mesoblast do not take part in the formation of the 

 blood and bloodvessels, as indicated above, but cells penetrate at the 



