250 TRANSACTIONS OF THE CANADIAN INSTITUTE. [VOL. II. 



Wenckebach* found that in Teleost embryos the blood cells originate 

 from a mass of cells placed under the notochord and between it and the 

 hypoblastic layer. The origin of this cell mass could not be determined, 

 when he published his first paper, but afterwards he traced it to the 

 mesoblast and was able, therefore, to corroborate Ziegler'sf first observa- 

 tions on this point. This intermediate cell mass may arise, as in Belone, 

 from an impaired organ but in the Salmon it is formed by the fusion of 

 two separate columns of cells. The blood cells are thus, according to 

 Wenckebach, of mesoblastic origin and are not derivable in any way 

 from the hypoblast or from the periblastic cells. 



ZiegleriJ: confirms Wenckebach's observations on the development of 

 the blood cells in the majority of Teleost embryos out of the cellular 

 elements of the intermediate cell mass placed between the entoderm 

 and chorda. This mass is of mesodermal origin and the cells con- 

 stituting it wander away over the yolk and, in a measure, as they do this 

 they make the cavities previously occupied by them larger and larger, 

 the cavities forming, finally, the cardinal veins. Up to this time the 

 blood which is free from cellular elements, flows in closed vessels 

 represented at this stage by the heart, aorta, caudal vein and sub- 

 intestinal veins. The latter empty on the yolk and the blood passes 

 from the posterior surface of the yolk sack to the heart, not in 

 a closed vessel, but free in the space between the yolk and the ectoderm. 

 There arises in the yolk a corresponding furrow to which wandering 

 cells pass to form a vascular wall. These wandering cells are in no way 

 distinguishable from the blood corpuscles of the same stage which are 

 abundant on the surface of the yolk and which arise, as already said, 

 from the elements of the intermediate cell mass. Sometimes, as in 

 the pike, a formation of blood cells, similar to that occurring in the 

 intermediate cell mass, obtains in a portion of the aorta. 



According to this view the blood cells are derived from the columns 

 of cells which occupy the position of the developing cardinal and other 

 veins and they are not, except accidentally, and through their amoeboid 

 movement, connected with the yolk. 



* The development of the blood corpuscles in the Embryo of Perca fluviatilis. Jour, of 

 Anat. and Phys. Vol. XIX., 1885, p. 231. Also : Beitrage zur Entwicklungsgeschichte der 

 Knochenfische. Arch, fiir Mikr. Anat., Bd. XXVIII, p. 225. 



* Die Embryonale Entwicklung von Salmo Salar. (Inaugural Dissertation), Freiburg, 1882. 



* Die Entstehung des Blutes bei Knochenfischembryonen. Arch, fur Mikr. Anat., Bd. 

 XXX, s. 596. Also : Die Entstehung des Blutes der Wirbelthiere. Berichte d. Naturforsch. 

 Oesell. zu Freiburg i. B. Bd. IV. s. 171. 



