THE MAIN VESSELS OE THE AREA VASCULOSA. 



99 



through omphalic arteries, Om. A., and thence to the capillaries of the area vas- 

 culosa and so on to the venous trunks again. As shown in the figure, which pre- 

 sents the under side of the area, the left omphalo-mesaraic vein preponderates, 

 and in the latter stages this difference becomes more marked, until finally the 

 right stem is- very inconsiderable in comparison with the great left vein. The 

 time at which the disparity commences is extremely variable, as is also the de- 

 gree of inequality between the two veins. 



The following description probably represents what was the primitive con- 

 dition of vessels in the mammalian area vasculosa. It applies to an early stage 



Fig. 45. — Area Vasculosa of a Rabbit, Presumably of about Twelve Days. — {After Van Beneden 



and Jtilin.) 



in the rabbit, which has been figured by Bischoff", whose figure is copied in Kolli- 

 ker's " Grundriss." An essentially similar arrangement of the vessels exists also 

 at a corresponding stage in the dog. The veins are much more symmetrical 

 than in the chick, and have the same general plan; the sinus terminalis belongs 

 to the venous system, so that the connection with the arterial circulation, found 

 later, is secondary; the aorta of the embryo is double, and gives off on each side 

 (segmentally arranged?) transverse branches, one of which develops into the 

 large trunk shown in figure 45; the network of small vessels forms two layers, of 



