63 



perty that these vessels of the area vasculosa ramifying over the yolk^ 

 as they did before the entoderm cells have been-yolk laden to that 

 extent, carry the reserve food toward the embryo". 



HuBRECHT sees "no reason for wonder that the surface of the um- 

 bilical vesicle should, during the embryonic period, play an active part in 

 this direction, if we remember how copiously blood formation is going 

 on during embryonic life in another derivative of the entoderm, viz., 

 the liver, not to mention the increased significance which we have to 

 ascribe to the entoderm as the primary source of blood and blood- 

 vessels, since the recent researches of Rückert and Mollier" (39). 

 Maximow, however, has shown that the blood cells appearing in the 

 liver do not arise from the entoderm cells, as urged by Janosik (22 

 and 23) and apparently accepted by Hubrecht, but from the connective 

 tissue (mesoderm — septum transversum) which these cells invade. 

 While Schridde vehemently denies the presence of mesenchyme in the 

 fetal human liver, he nevertheless, together with others, derives the 

 blood cells from the capillary walls of the liver (also Schmidt). An 

 entodermal origin for blood cells in the liver seems definitely disproved. 



Moreover, in the two well-preserved umbilical vesicles here con- 

 sidered no giant-cells (supposed ancestors of normoblasts, Saxer) were 

 found arising from the entodermal cells. The entoderm whether in 

 tubules, cords or masses is always distinctly marked off from the 

 mesenchyme in which blood cells are seen to arise. Absolutely no 

 evidence appears here for an origin of blood copuscles from ento- 

 derm, either in the umbilical vesicle or liver. 



There remains little question that the mesoderm has a dual origin 

 from the primitive streak (ectoderm) and primitive hypoblast ("proto- 

 chordal plate" and "annular zone" — Hubrecht) in many of the verte- 

 brates as described by Rückert and Mollier. Hubrecht has adduced 

 quite convincing evidence in favor of this point also in the higher 

 mammals, including the hedgehog and Tarsius. But the matter 

 cannot yet be said to be elucidated for apes and man. Moreover, 

 once the mesoderm is formed, from whatever source, blood is not 

 conclusively shown to arise from entoderm. The evidence derived 

 from a study of the human umbilical vesicle, with reference to the 

 blood islands is strongly against a partial entodermal origin of the 

 blood. 



Apart from other facts of early mammalian ontogeny which in- 

 dicate the primitive character of Primates, the structure of the yolk 

 sac suggests more forcibly a condition of secondary modification of a 

 functionally more significant structure in probably reptilian ancestors 



