29 



organ. 7) One naust possibly admit that "when the function disappears, 

 the organ also disappears". But this is exactly what happens in the 

 case of the human umbilical vesicle. It is an embryonic organ (probably 

 essential to the developing embryo in the same sense as the noto- 

 chord and Meckel's cartilage) with an important function. 



The umbilical vesicle resembles rudimentary organs such as the 

 hypophysis, epiphysis and the appendix in the sense that it has lost 

 one of its phylogenetically earlier functions and has retained another. 

 It has decreased in size, but its essential elements have not atrophied. 

 Just as the hypophysis and appendix have lost their phylogenetically 

 earlier functions of reception of nerve stimulus and absorption of di- 

 gested food respectively, and have retained another, that of secretion 

 (internal and mucous respectively), so the yolk-sac may have lost its 

 function of yolk assimilation but has retained that of hematopoiesis (and 

 also that of forming a probably useless secretion). Once its function 

 is performed — once the liver can carry on the work of blood-cell for- 

 mation — the vesicle disappears. It decreases in size and its elements 

 atrophy and the organ dwindles away, just as Branca would have it 

 for a rudimentary organ. The human umbihcal vesicle is not a ru- 

 dimentary organ in the extreme sense of Branca's definition, but it 

 need not therefore necessarily have the full complement of functions 

 of homologous tissues. Branca's efforts to establish an analogy be- 

 tween the human umbilical vesicle and the intestine involve speculations 

 that appear to overreach the limits of legitimate inference. 



It remains to note in this connection that cells resembling the 

 polynuclear "Riesenzellen" described as arising from the entodermal 

 cells in the human umbilical vesicle by Spee (41) have not been seen 

 extravascularly anywhere in my specimens. Nor is there any evidence 

 that the blood cells have any close genetic relation to the entoderm, 

 a point which will be more fully discussed below. Maximow (32) has 

 described similar giant cells arising from the mesenchyme of several 

 mammalian yolk-sacs; viz, rabbit, cat, guinea-pig, ra;t, mouse and dog. 

 These cells can also be demonstrated in the capillaries of the um- 

 bilical vesicle and in the liver and heart of the 13 mm human spe- 

 cimen. What evidence appears respecting the origin of the first blood 

 cells points unequivocally to the mesoderm, and to this extent agrees 

 with the findings of Maximow in support of a monophyletic ancestry. 



Obviously nothing can be determined here regarding the origin 

 of the mesoderm, whether from the ectoderm exclusively or from both 

 the ectoderm and entoderm. The later investigations of Rückert and 

 MoLLiER (39), HuBRECHT (21) and Assheton (1, 2) point to a double 



