52 VASCUliOGENESIS IN THE CAT 



of these cords and the principle on which they are inter])reted as in- 

 growths. Continuity of structure at a given period of development can 

 not be held to establish the presence of continuity at earlier stages, for 

 it may be secondarily achieved, as is sho-wn by the union of the meso- 

 nephroic tubules with the pronephric duct, to instance but one of sev- 

 eral similar junctions in the development of the genito-urinary tract. 

 In this particular case I must consider both the assumption of primitive 

 continuity and of ingrowth wholly iuA'alidated bv the experiments of 

 Hahn (Arch. f. Entw. Mech. I "Organismen Bd. 27, 1909) and the 

 recent very satisfactory confirmatory results of Miller and Mc- 

 Whorter, which after some amusing editorial adventures in the 

 office of the American Journal ot Anatomy were presented at the 

 thirtieth session of the A.A.A. (Proc. Am. Ass. Anat., Anat. Rec, vol. 

 8, no. 2, p. 91). As to the nature of the cellular cords designated angio- 

 blast, it is to be noted that Bremer does not figure or describe either 

 mesostroma or mesenchj^me in his embryos. As he does not distinguish 

 these formations, it is possible that they are included in his angioblast 

 cords, which would then represent an early for the most part ]3re- 

 vascular stage of the splanchnopleuric mesenchyme. I am inclined 

 further to this view by the loss of cords in the embrj-o of 6 to 7 seg- 

 ments as evidenced in the comjjarison of figures 1 and 3, and to which 

 Brenner calls attention in his text. The most plausible explanation 

 would be that they had resolved themselves into mesenchyme; that 

 therefore the primitive network was mesenchyme, which in widely 

 separated regions was partially transforming itself into endotheliiun. 



I am interested to note that in his second paper, a highly interesting 

 study of early vessels in man (Proc. Am. Ass. Anat. 30th Sess. Anat. 

 Rec, vol. 8, no. 2, p. 97) Bremer abandons his original position and 

 accepts the development of discontinuous vascular anlages independent 

 of the vessels of the yolk-sac, but nevertheless retains the principle 

 of growth in continuity' in the vascularization of the embryonic body. 

 With the general principles of blood vessel origin from mesoderm I am 

 in hearty agreement, as also with the multiplicity of anlages, and their 

 intimate relation to the coelomic mesoderm. In these points there 

 now seems to be a very general agreement between the students of 

 mammalian vasculogenesis. But I must question the validity of a 

 continuous angioblast network "composed of angiocysts and solid 

 cords," which appears superfluous in view of Bremer's conclusion that 

 vessels "originate from ingro^vths of mesothelimn as a nmnljer of sepa- 

 rated cords, angiocysts, or blood islands, which are soon connected by 

 sprouts of endothelium." In the interpretation of the solid strands 

 connecting angiocysts as 'angioblast' Bremer states that he relies on 

 his earlier work upon the ralibit, and I would again express my dissent. 

 The mesenchyme and mesostroma form a protoplaismic continuimi in 

 which are contained angiocysts. To designate such portions of it as 

 intervene between two angiocysts "angiolilast" is in my opinion to make 

 a distinction which does not exist, and which would not suggest itself 

 in regions where the mesenchjTne preponderates over the blood vessels 

 as in the somatopleure. 



