186 W. B. CHAPMAN 



Figure 4 is a drawing of an unoperated chick which had been 

 incubated 48 hours and had developed abnormally. The embryo 

 appears as an oval membrane with two little splotches of proto- 

 plasm underneath it. There was no circulation, and a fairly uni- 

 form capillary plexus was present throughout the area, extending 

 under the oval membrane. Only at one place — anterior to the 

 membrane — is there any sign of a vessel larger than a capillary, 

 which is very probably the rudimentary heart and anterior vitel- 

 line veins. There is no trace of any vessel which could be inter- 

 preted as omphalo-mesenteric artery. It will be noted that, as in 

 figure 3, the continuity of the plexus has begun to be lost in the 

 area to the left of — and posterior to — the embryonic rudiment. 



In all the operated chicks which were studied, by injection and 

 by combined injection and staining, there was never found any 

 definite vessel which could be interpreted as omphalo-mesenteric 

 artery or vein, or posterior vitelline vein. There were, however, 

 certain characteristic changes, which were repeated in all speci- 

 mens, some of which resembled the changes in normal chicks, 

 and which will be taken up separately. 



The anterior vitelline veins 



As shown in figure 1, there is a space between the right and 

 left anterior vitelline venous plexus at the time circulation com- 

 mences. In normal chicks, as PopofT has shown, this space is 

 encroached upon by the two plexus, until it is obliterated, the 

 two veins fusing in the mid-line, forming a single vein, which, 

 for a time, returns most of the blood of the extra-embryonic 

 area. It was most interesting to find that in the absence of a 

 heart-beat, the vessels in the proamnionic region continue to de- 

 velop in an almost normal manner for a time. During the second 

 day after the removal of the heart (third day of incubation) the 

 right and left vitelline veins fuse across the mid-line anterior to 

 the embryo, and a single vessel, which corresponds to the left 

 anterior vitelline vein of the normal chick, is formed (figs. 5, 6 

 and 7, cf. also fig. 3). During the fourth day, however, this an- 

 terior vessel begins to break up into capillaries and soon disap- 

 pears. This is the only vessel larger than a capillary that is 



