46 VASCULOGENESIS IN THE CAT 



Caudad the anlages of the umbilical line are in the wall of the 

 amnio-embryonic cavity (figure 18). The amnionic position 

 of vascular rudiments has been described by Bonnet, Spec, and 

 Keibel. 85 Although the position of the umbilical in the cat is 

 really at the amnio-embryonic junction, and appears to be un- 

 usual only because of the moderate entypy of the blastoderm 

 and the early closure of the amnion caudad, there are in addition 

 endothelial sacs actually in the amnion itself as may be seen in 

 figure 19 from a 16 somite embryo. 



The foregoing data seem fully to warrant the conclusions that 

 endothelium is formed directly from mesenchyme in the soma- 

 topleure and that vessels result from the concrescence of discrete 

 anlages. With these facts in mind we may approach the more 

 difficult region of the splanchnopleure. 



THE EARLY FORMATION OF VESSELS ABOVE THE ENTODERM 



The formation of mesenchyme and endothelium in the splanch- 

 nopleure is essentially the same process as in the somatopleure; 

 the differences are mainly in the proportions of the two tissues 

 for here endothelium predominates and is associated with the 

 extresive production of blood cells. In brief, there is a separation 

 of cells from the mesoderm, by individual migration and exten- 

 sively in groups and masses; these elements are interposed between 

 the entoderm and the compact mesoderm, that is they come to 

 occupy and finally obliterate the hyperentodermal space. The 

 major part give rise to endothelium, and in the case of the larger 

 masses (blood islands) to blood cells. Of the remainder, a con- 

 siderable proportion subsequently become endothelium, but there 

 is always a residuum of mesenchyme present. Later this becomes 

 conspicuous and surrounds and embeds the vessels. 



The process is initiated by the formation of blood islands. 

 These appear as spindle shaped discrete thickenings of the meso- 

 derm at a considerable distance lateral and caudal to the embryonic 

 axis, in regions where the mesoderm is free and separated by an 

 interval, probably much increased by fixation, from the entoderm. 



85 Quoted from Ruckert, loc. cit., pp. 1253-4. 



