ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 245 



definitive!! oder sekundiiren" (p. 489). He described the first or primitive blood- 

 cells as arising within the vessels in connection with endothelium, like those of birds. 

 These primitive erythroblasts he believes die out completely and are replaced by 

 secondary red cells, which arise from indifferent mesenchyme cells outside of vessels. 

 These mesenchyme cells become lymphocytes, which in turn give rise to erythro- 

 blasts (p. 547). 



In the article of Mollier (1909), in which he brings out the idea of the extra- 

 vascular origin of erythroblasts in the developing liver, certain figures (e. g., figs. 

 7 and 8) can be interpreted easily as a solid mass of angioblasts in which the process 

 of liquefaction is going on with the production of hemoglobin-bearing cells. Indeed, 

 this interpretation also fits in with his conclusions (p. 519) : 



"In der embryonalen Leber der Saugetiere werden Blutzellen gebildet aus einem 

 indifferenten Material, dem Reticulum, das vom visceralen Blatt des Mesoderms gebildet 

 sich zu Endothelien, Blutzellen und Stiitzgewebe differenziert. Die Blutzellenbildung 

 erfolgt ausserhalb der Gefasslichtung im Reticulum. Die blutbildenden Gefassanlagen 

 haben alle reticulare Wand. Die Blutzellen wandern nicht selbstandig durch geschlossenes 

 Endothel in die Blutbahn ein, es reisst das Endothel zu diesem Zwecke auch nicht ein, 

 sondern es bleibt die retikulare Gefasswand solange bestehen, als die Blutbildung anhalt." 



In studying sections of the developing liver in very young pig embryos, it 

 seems to me that one can identify solid masses of cells between the columns of liver 

 cells as analogous to the solid masses of angioblasts to be seen in blastoderms 

 of the chick. These are the solid, blood-forming capillaries in the sense of O. Van 

 der Stricht. Thus, in view of the great discrepancy in regard to the formation of 

 erythroblasts, namely, that in birds they form within the lumen of vessels and 

 in mammals outside of the lumen, it seems important to test the identification of 

 angioblasts in the case of mammals, especially in connection with Mollier's observa- 

 tion that the walls of vessels in the liver remain reticular as long as blood is being 

 produced there. The question is, therefore, can it be shown for mammals that cells 

 which may be identified as angioblasts produce erythroblasts, and that associated 

 with the process there is a certain destruction, liquef action, or vacuolation (see 

 Mollier's text-figures 7 and 8) of cytoplasm, such as can be determined for birds? 



To return to the specimen shown in figure 6, plate 2, the area pellucida shows 

 three zones: anterior, middle, and posterior. These zones are more striking still 

 in the other photographs on plate 2, especially in figure 7. Over the anterior zone 

 the vesicles of the amnio-cardiac cavities have become very large, with fine, sharp 

 boundaries. Besides these boundaries there are numerous small, isolated clumps 

 of angioblasts. The photograph does not enable one to distinguish these two 

 structures, but they would not be confused in the specimen. Over this area angio- 

 blasts are much more scanty than farther posterior, and hence it always happens 

 that they remain very much longer as isolated masses. This is simply due to the 

 fact that the distances between the masses are greater and hence it takes longer 

 for the sprouts to bridge the gaps. When the vacuolation occurs in these isolated 

 clumps there results the formation of isolated vesicles which may be found any- 

 where in the area pellucida, but more commonly in this region. They were noted in 



