110 THE THIRD DAY. [CHAP. 



ing about half-way towards the pole of the yolk opposite 

 to the embryo. 



The vascular area, though still increasing, is much 

 smaller than the total opaque area, being in average- 

 sized eggs about as large as a florin. Still smaller than 

 the vascular area is the pellucid area in the centre of 

 which lies the rapidly growing embryo. 



During the third day the vascular area is not 

 only a means for providing the embryo with nourish- 

 ment from the yolk, but also, inasmuch as by the dimi- 

 nution of the white it is brought close under the shell 

 and therefore fully exposed to the influence of the 

 atmosphere, serves as the chief organ of respiration. 



This in fact is the period at which the vascular area 

 may be said to be in the stage of its most complete de- 

 velopment; for though it will afterwards become larger, 

 it will at the same time become less definite and rela- 

 tively less important. We may therefore, before we 

 proceed, add a few words to the description of it given 

 in the last chapter. 



The blood leaving the body of the embryo by the 

 vitelline arteries (Fig. 36, R. Of. A., L. Of. A.) is 

 carried to the small vessels and capillaries of the vascu- 

 lar area, a small portion only being appropriated by the 

 pellucid area. I 



From the vascular area part of the blood returns 

 directly to the heart by the main lateral trunks of the 

 vitelline veins, R. Of, L. Of During the second day 

 these venous trunks joined the body of the embryo 

 considerably in front of, that is, nearer the head than, 

 the corresponding arterial ones. Towards the end of 

 the third day, owing to the continued lengthening of 



