30 



origin also for many mammals, as seems definitely demonstrated for 

 ichthyopsida and sauropsida. 



Part II. 

 The Blood Islands — Description. 



The blood islands are confined solely to the mesenchyme. There 

 exists a clear demarcation between them and the entodermal tubules 

 and cords. They are present only in the distal portions of the vesicle 

 of the older embryo. Slide number 4 contained them most abund- 

 antly. They were entirely absent in the vesicle of the 9,2 mm embryo. 

 This seems at first thought a strange fact. But variations in respect 

 to the presence of blood islands are really no more singular than 

 variations in respect to size, number of tubules, etc. in vesicles of em- 

 bryos of the same length. The age of the vesicle may have com- 

 paratively little to do with the age of the embryo. This is evidenced 

 also by the fact that occasionally the umbilical vesicle is found intact 

 and in living condition at birth (Schultz, 42). 



The embryo and its umbilical vesicle seem to be largely indepen- 

 dent of each other after the earliest stages. Even its hematopoietic 

 function may conceivably be simply assistive rather than originative 

 and essential, since mesenchyme in the embryo is potentially indifi"erent, 

 and, as shown by Maximow, gives rise in mammals to identical pri- 

 mitive blood cells whether in the yolk-sac, liver or elsewhere (e. g. 

 head mesenchyme). 



That the human umbilical vesicle has a hematopoietic function is 

 indisputable. But whether this is absolutely essential in the sense 

 that in its absence there would result a complete absence of blood-cell 

 progenitors can obviously not be determined on the basis of results 

 of anatomic studies. This point awaits experimental evidence. In 

 view of the work of Maximow it seems unlikely i). That its he- 



1) Walter E. Dandy, in a recent study of "A Human Embryo 

 with Seven Pairs' of Somites" (Amer. Journ. of Anat., Vol. 10, 1910, 

 No. 1, p. 85 — 108), details additional evidence which indicates that the 

 umbilical circulation precedes the vitelline, as described by Eternod 

 for a 1,3 mm embryo. In both embryos blood islands are present but 

 remain apparently unconnected with the chorionic circulation by vitelline 

 vessels. The source of the blood in the embryonic vessels is supposed 

 to be the proliferating endothelium of capillaries in the chorionic mem- 

 brane, the yolk sac coming to function as a hematopoietic organ only 

 secondarily. The absence of capillaries in the body and their presence 

 in the chorion, embryophore and umbilical vesicle in embryo Frassi 

 (Archiv f. mikrosk. Anat., Bd. 71, 1908) indicates farther that the early 



