EARLIEST BLOOD-VESSELS IN MAN 449 



hilufungen, welchc sich stellenweise in Kreisform zu lagern, und ein 

 lAimen in ihrer ]\Iitte frei zu lassen scheinen. Auch findetman ahnliche 

 Gebilde in den seitlichen Partien des Haftstieles, allein ich wage nieht zu 

 entscheiden, ob es sich hier etwa um die ersten Gefassanlagen handelt. 

 Jedenfalls sieht man nirgends in diesen kreisformigen Gebilden einen 

 Inhalt, geschweige denn etwa Dinge, die an Blutkorperchen erinnern 

 konnten. 



In his drawing (Fig. 17) these rings of cells are shown both on 

 the yolk-sac and on the body-stalk. Herzog (p. 373) finds similar 

 appearances ''around the allantois stalk where its mesoderm is 

 continuous with the yolk-sac mesoderm." ''These formations 

 undoubtedly represent the earliest anlagen of the yolk-sac blood- 

 vessels.'' Herzog's interpretation of these cellular rings I have 

 found to be incorrect, but his mention of them helped to point 

 to the location of the blood-vessel anlages. 



The embryos especially studied in this investigation are: (1) 

 one of about 1 nun. (no. 825 of the Harvard Embryological Collec- 

 tion, fixed in Zenker's fluid, 10 y. sections cut in paraffin, stained in 

 borax carmine and orange G) ; (2) Grosser's embryo, of practically 

 the same age as the preceding (fixed in picric-sublimate, 10 /x 

 sections cut in paraffin, stained in paracarmine) ; and (3) Herzog's 

 embryo (fixed in Zenker's fluid, 7 /x sections cut in paraffin, stained 

 ill hematoxylin and eosine). 



Beside these three I have examined the embryos described by 

 Debeyre, Frassi,*' Dandy, ^ Bryce-Teacher,^ and many others.-' 



In the Minot embryo, by the graphic reconstruction method, 

 it is easily seen that on the yolk-sac, between the entoderm and 

 the mesoderm, there is a net of vascular tissue, one-layered and 

 consisting of l)lood-islands, solid cords and angiocysts. This net 

 is best developed, and the blood-islands in it are located exclusively, 

 in the hemisphere of the yolk-sac opposite the embryonic shield; 

 this fact was pointed out b}^ Minot in his article on the development 



« P>assi, L. Arch, inikr. Aiiat. B<1. 70,. p. 492, and Bd. 71, p. G(i7, 1907-08. 



^ Dandy, W. E. Am. Jour. Anat. Vol. 10, p. So, 1910. 



^ Bryce, T. H. and Teacher, J. H. Memoir, 190S. 



' I wish to express here my most sincere thanks to those who have given me 

 ready access to much of the material used cither for the substance of this paper or 

 for valuable comparisons — to Professors Bryce, Dobeyro, (Jrosser, Keibel, KoU- 

 iiiann, Midi, Robert Meyer, and Teutdier. 



