CIRCULATION IN ARTICULATED ANIMALS. 



311 



of the solid matter which it contains, the fluid is collected again by 

 other trunks, which convey it back to the point from which it started ; 

 there it is mingled with the fluid that has been newly absorbed, and 

 with that which has undergone aeration ; and it is then distributed, as 

 before, through the general capillary network of the body. 



551. Now this is very much the condition of the Human embryo, at 

 the time when vessels are first developed in its substance. These ves- 

 sels are formed by the coalescence of cells ; and from the contents of 

 these cells, which have been imbibed from the yolk, the first blood seems 

 to be derived. The first formation of blood-vessels takes place, not in 

 that part of the embryonic structure which is to be developed into the 

 perfect animal, but in a membranous expansion from it, which surrounds 

 the yolk, and which answers the purpose of a temporary stomach. A 

 capillary network is formed in a limited portion of this membrane, 

 termed the vascular area (Fig. 88) ; and this not by the branching of 



Fig. 88. 



Vascular area of Fowl's egg, at the beginning of the third day of incubation; a, a, yolk; ft, b, b, b, venous 

 sinus bounding the area; c, aorta; d, punctum saliens, or incipient heart; e, e, area pellucida ; /,/, arteries 

 of the vascular area; g, g, veins; h, eye. 



larger trunks, these trunks being subsequently formed by the reunion 

 of the capillaries. The first movement of the blood is towards the cen- 

 tral spot, in which the organs of the permanent structure are being 

 evolved; and it takes place before the incipient iieart has acquired any 

 muscularity, so that it must be quite independent of any contractile force 

 exerted by that organ. Here too, then, we perceive that the circulation 

 is essentially capillary ; and that it is sustained by forces^ very different 

 from those, of which the action is most evident to us in the higher 

 animals. 



552. As we ascend the animal scale, however, we find that provision 

 is made for a more regular and vigorous Circulation of the Blood, than 

 that which exists in the lowest classes. Even in the class of Echino- 

 dermata (including the Star-fish and Sea-urchin), a portion of the prin- 

 cipal vessel is peculiarly endowed with contractile power ; and this may 

 be seen in constant pulsation, like the heart of the higher animals, 

 alternately contracting, to propel the fluid it contains, through the ves- 



