August 4, 1916] 



SCIENCE 



155 



nucleoli. Again Kampmeier 20 has shown 

 that both venous and lymphatic endothe- 

 lium in the toad can be distinguished from 

 mesenchyme at certain stages by the pres- 

 ence of a greater number of yolk globules. 

 Indeed, this differentiation of vascular and 

 lymphatic endothelium from the mesen- 

 chyme was so striking as to convince 

 Kampmeier that the lymphatics arose from 

 the veins, though he had previously held the 

 view that they arose from tissue-spaces. 



These observations, valuable as they are, 

 are not sufficiently universal to determine 

 the nature of endothelium. The lymphatic 

 endothelium grows from the endothelium 

 of the veins; but since it varies slightly 

 from the venous endothelium we may say 

 that it is secondarily differentiated from it. 

 This idea leads us directly to the most 

 fundamental problem connected with the 

 entire vascular system — namely, how does 

 endothelium arise, how do the first endo- 

 thelial cells differentiate? The question 

 of the origin and the growth of the lym- 

 phatic system will not be completely 

 solved until its essential tissue endothelium 

 is completely understood. This leads us to 

 seek for the origin of the first blood-vessels. 



The question of the origin of the heart 

 and blood-vessels has a vast literature. 

 Since the time of Wolff and Pander, it has 

 been known that blood-islands in the chick 

 arise in the wall of the yolk sac. Then 

 His 21 discovered that blood-vessels arise by 

 a differentiation of vaso-formative cells or 

 angioblasts. This is the fundamental point 

 which recent work confirms, His having 

 proved that angioblasts differentiated in 

 the wall of the yolk-sac, and having seen 

 that they did invade the embryo, advanced 

 the hypothesis that all the angioblasts dif- 

 ferentiated in the yolk-sac and then in- 



20 Kampmeier, O. F., Amer. Jour, of Anat., Vol. 

 17, 1915. 



21 His, W., TJntersuehungen fiber dir erste Anlage 

 des Wirbelthierleibes, Leipzig, 1868. 



vaded the body from the embryonic mem- 

 branes. The theory regarding angioblasts 

 thus became centered around the idea of 

 this invasion, and the more fundamental 

 point was obscured. In recent years this 

 theory that all of the vessels of the embryo 

 are derived from the vessels of the mem- 

 branes has been disproved by certain ex- 

 periments of Hahn. 22 Hahn selected chicks 

 in the stage of the primitive streak and 

 burned out the membranes opposite the pos- 

 terior end of the streak. In a few specimens 

 which lived he found a small aorta and car- 

 dinal veins on the injured side of the em- 

 bryo. These results have been confirmed by 

 Miller and McWhorter 23 and by Reagan 24 

 on the chick and again by studies on the 

 fish embryo by Stockard. 25 It may thus be 

 regarded as proved that blood-vessels arise 

 both within the embryo and in the embry- 

 onic membranes. 



Stockard then went on to attack the more 

 fundamental problem, how does endothe- 

 lium first arise? In studies made on the 

 yolk sac of the living fish embryo, he found 

 that endothelium arises as spindle cells 

 which differentiate out of mesenchyme. 

 Moreover, he found that the endothelial 

 cell was distinct from the blood-cell. This 

 confirmation of the angioblast of His I re- 

 gard as a very important contribution. 



It is very clear in following the work of 

 His, that he made studies on the living 

 blastoderm of the chick, but so far as I am 

 aware McWhorter and Whipple 26 were the 

 first to study the living blastoderm of the 

 chick in a hanging-drop preparation. By 



2 - Hahn, H., Arch, f . Entwieklungsmechanik der 

 Organismen, Bd. 27, 1909. 



£3 Miller and McWhorter, Anat. Becord, Vol. 8, 

 1914. 



2* Keagan, F. P., Anat. Becord, Vol. 9, 1915. 



as Stockard, C. B., Amer. Jour, of Anat., Vol. 18, 

 1915. Two articles. 



26 McWhorter and "Whipple, Anat. Becord, Vol. 

 6, 1912. 



