182 W. B. CHAPMAN 



up into a plexus of venous capillaries through which, at a later 

 stage, the blood from the sinus is returned to the heart. This 

 venous plexus gradually becomes denser and evolves into the 

 primitive right and left anterior vitelline veins as the heart is 

 approached. Near the posterior end of the embryo, each aorta 

 breaks up into capillaries, which are continued outward onto the 

 extra-embryonic area by a dense capillary meshwork, in which 

 there is as yet, no vessel marked out, as the omphalo-mesenteric 

 artery — which develops here soon after circulation commences. 

 There are the well-known 'clear' areas, without blood capillaries, 

 surrounding the head region, others on either side of the body of 

 the embryo between the anterior vitelline yeins and the plexus 

 in the omphalo-mesenteric region, in which, however, a single 

 very narrow capillary is present in the specimen drawn, while 

 there is a large oval non- vascular region including and surround- 

 ing the caudal undifferentiated end of the embryo. The capil- 

 laries bordering the clear areas next the body of the embryo are 

 especially narrow, while near the border vein, particularly in the 

 posterior portion of the area, they are very wide. This condi- 

 tion represents that immediately before the effects of circulation 

 begin to be exerted, for, as stated in the note, circulation had 

 already started on the other side, and a suggestion of the om 

 phalo-mesenteric arteries could be made out there. 



In normal development, soon after circulation commences, the 

 omphalo-mesenteric arteries differentiate out of the capillary 

 plexus, the anterior vitelline veins grow together anterior to the 

 embryo, forming a single large vein, and, somewhat later, the 

 posterior and omphalo-mesenteric veins develop. The fate of 

 the border vein in normal embryos will be referred to later. 

 These changes have been fully described by Thoma ('93) and 

 Popoff C94). 



Let us now turn to the fate of the vessels deprived of the ac- 

 tion upon them of the factors concerned with the circulation. 



minutes before granules of ink began to appear in the left aorta. In a number of 

 embryos studied, the circulation was invariably established upon the right side 

 first. It will be noted that the aortae pass immediately into the capillary plexus 

 in the posterior portion of the embryo, and that no large branches have as yet 

 been formed out upon the jolk-sac. 



