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 Jitlht.} 



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 



