13 



view of this condition it has seemed worth while to give a detailed 

 description of the microscopic structure of a single wellpreserved 

 umbilical vesicle just past the phase of maximum development, and 

 in the light of these facts to suggest the most probable functional 

 interpretation. The points of special interest concern the entodermal 

 tubules of the wall, and the blood islands. The stage of develop- 

 ment here studied presents probably the most important phase of its 

 entire history, as will appear more clearly below. 



As far as I have been able to learn the blood islands have never 

 been specifically described for the human umbilical vesicle, though 

 they have been incidentally mentioned by a number of writers including 

 more particularly Graf Spee (46) and Meyer (33). Hubrecht (21) 

 also mentions them in descriptions of the early ontogeny of the hedge- 

 hog, shrew and Tarsius; and Selenka (45) notes them for apes. 

 Schridde (44) in a recent communication speaks of "Bluträume" in 

 the embryophore and umbilical vesicle of a 1 mm human embryo. 

 These are said to be filled with only one kind of cell, the primary 

 erythroblast (two to three times the size of a postembryonic erythro- 

 cyte). In older vesicles (from seven embryos ranging from 2,5 mm to 

 13 mm) the blood system is described as a "Gefäßnetz", and the 

 primary erythroblasts, the only cells found in the vessels, are said to 

 arise from the "Gefäß-Wandzellen". 



Reasoning from analogy, most of the earlier anatomists ascribed 

 nutritive and hematogeneous functions to the yolk sac. Its nutritive 

 importance is now generally denied not only for man but also for 

 apes and ruminants (Selenka, 1891). The more correct designation, 

 as HuBRECHT suggests, in these cases is "umbilical vesicle". 



On the basis of a similarity of form and content between the 

 cells of the entodermal masses and tubules of the umbilical vesicle 

 and the cells of the liver, Spee (46), Paladino (35) and Saxer (40) 

 are inclined to attribute to the vesicle a hepatic function. This as- 

 sumption is invalidated by the fact that the liver cells are already 

 abundant when the entodermal cells have attained their greatest 

 development and are arranged in the peculiar liver-like tubules and 

 cords. 



Branca (6), who has recently made a careful histologic study 

 of three human umbilical vesicles (length of embryos, 5 mm, 9 mm 

 and 11 mm respectively) urges an additional objection to this hypo- 

 thesis mainly on grounds of a dissimilarity in structural details; e. g. 

 the presence in the umbilical vesicle of connective tissue between the 

 entodermal cells and the capillaries; the presence of blind and solid 



