44 VASCULOGENESIS IN THE CAT 



the aorta, in the former the umbilical angiocysts are enlarging. 

 The distending fluid must be derived from the adjacent region 

 and its determination to these reservoirs must set up currents in 

 the neighborhood. It is suggested that the conformation of the 

 region (fig. 15) is favorable to the direction of such a current 

 along the dorsal surface of the lateral mesoderm in the narrow 

 interval between it and the bulk of the somatopleuric mesenchyme 

 and that to this current the vasofactive cells owe their trans- 

 verse orientation. Mesal to the coelom where the nephrotomes 

 are resolving and where the current, if there be one, is directed 

 ventro-mesad towards the aorta, the vasofactive cells have an 

 oblique position. I have already referred to Bartels' view that 

 the addition of cells to the tips of lymphatics is occasioned by 

 currents; it would be satisfactory to think the same principles 

 at work in both late and early stages of vascular formation and to 

 find that the current, the sine qua non of the persistence of endo- 

 thelium, is also its determining cause. 



The vasofactive cells are dorso-ventrally flattened, they occur 

 singly, in groups of two or three, continuous with, or wholly sepa- 

 rate from the angiocysts. Where joined to the latter they could 

 of course, in section be taken for sprouts, in spite of their flat 

 plate-like form. But if more caudal regions of the same embryo 

 are examined, or a corresponding region in a younger embryo, 

 isolated flattened cells are found constantly, which I take to be 

 evidence that the continuous plates, or if you will, cords are 

 built up by the accretion of separate cells. When it is borne in 

 mind that the distance from the umbilical to the cardinal line 

 can be spanned by a half dozen of these cells, the opportunity of 

 finding the angiocysts of these lines, continuous by means of strands 

 of 'angioblast/ is not wanting. In one case (an embryo of 16 

 somites, Columbia Collection No. 551) I found such a cord ex- 

 tending from the umbilical vein to the aorta, which could have 

 been taken to prove the origin of the aorta from the umbilical 

 vein or vice versa by one who was apt at interpretation and had 

 refrained from examining earlier stages. 



In the 14 somite embryo (Columb'ia Collection No. 188) the 

 umbilical vein has become organized in its whole length; there 



