DEVELOPMENT OF THE FECUNDATED EGG. 511 



was longitudinal, now assumes the form of the letter S (Fig. 

 146, 4), ami commences to contract and propel the blood into 

 the peripheral vessels. 



These peripheral vessels, as we have already said, are 

 formed in their proper places and consist, at the first, of two 

 aortic arches, which are offshoots from the anterior extremity 



Fig. 146. Primary circulation.* 



of the cardiac tube. These curve around and below tlio 

 cephalic hood (anterior vertebral arteries}, unite in a single 

 trunk (aorta) ;it the median portion of the vertebral column, 

 and again divide, descending towards the caudal extremity 

 of the embryo by two branches, the posterior vertebral; 



* (terminal area of an embryo; the ventral surface of the embryo is pre- 

 sented. I, Terminal sinus. '2, Omphalo-mesenteric vein. 3, Its posterior 

 branch. 4, lie-art in the form of an S. 5, Primitive aorta, or posterior vertebral 

 arteries. (I, Omphulo-mesentene arteries. (Uischoff, " Developpement tie 

 rilmume," p. Ixiv.) 



