184 JOHN LEWIS BREMER 



on growing lymphatic vessels, and as those found by injections 

 of blood vessels by Evans (2) and others. It is my idea that 

 endothelium normally sends out sprouts at right angles to the 

 main stem at fairly regular intervals, in the same way that 

 the main stem of some trees sends out branches, and that in con- 

 sequence the endothelial mesh throughout its growing portion 

 is originally of a definite size. As this size is increased by the 

 general growth of the body, carrying the capillaries further and 

 further apart, new branches are given off between the old ones in 

 order to regain the normal mesh size. This applies equally to the 

 aorta, the circumference of which is, with growth, successively 

 large enough to accomodate more and more branches with the 

 normal interval between them, until, at the age when the meso- 

 dermal coat develops and stops further outgrowths, there are 

 perhaps twelve or fourteen such branches, equally spaced, around 

 the circumference. In the vertical plane there may be two or 

 three sets of branches to each segment before the advent of the 

 mesodermal coat. The later growth of the aorta, which may be 

 greater in one part than in another, would separate these branches 

 further from each other, but no new intervening vessels could be 

 formed. 



An anastomosing periaortic net having many connections 

 with the aorta and linking the earlier aortic branches would 

 result. Portions of such a net can be found occasionally, as 

 certain of the accompanying figures show; part has already been 

 injected by Jeidell, as already mentioned. Normally, the greater 

 part of it degenerates very early. That it has not been more 

 fully injected I attribute to the fact, as already suggested by 

 me (10), that new endothelial vessels may be solid cords, even 

 while connecting with hollow vessels or vesicles. Tracing such 

 cords in serial sections and amongst mesenchymal cells is ex- 

 tremely difficult, though portions of the plexus, but not its con- 

 nections, are recognizable in almost all the young embryos I 

 have examined. Even the point of origin of the smaller roots 

 from the aorta is hard to find, and I imagine that in many cases 

 they may be absent in the prepared specimen because they have 

 retracted from faulty preservation or under the influence of the 



