14 



tubules; and the absence of intercellular canaliculae. It must be 

 admitted, however, that the early developmental stages of the liver 

 are very like the conditions known to obtain in the vesicle of a young 

 embryo (2 to 10 mm). In the first instance tubules and columns of 

 eutodermal cells of the primitive gut invade the vascular connective 

 tissue of the septum transversum; in the second case tubules and 

 columns of entodermal cells of the lining epithelium of the um- 

 bilical vesicle invade the vascular connective tissue layer of its 

 wall. The difference in details appears to be one more of degree 

 than of kind, and cannot be urged as proof against at least a 

 glandular function. Though no evidence appears in support of 

 a hepatic function, one is not limited to the position held by 

 Branca that "La vesicule est une membrane de resorption, comme 

 I'intestine" as will be shown below. While the umbilical vesicle must 

 probably be regarded as largely a vestigial remnant of a once important 

 organ, evidence will be brought to show that the entodermal cells are 

 still transiently secretory and the mesenchyme actively hematopoietic. 



Moreover, militating against the idea that the presence of glycogen, 

 believed to be present by various workers (Paladino), indicates a 

 hepatic function, is the evidence produced by Gage (13) that the cells 

 of most embryonic tissues contain some glycogen. 



Nor does the presence of giant cells in the liver and umbilical 

 vesicle necessarily signify similarity of function as urged by Spee (47) 

 since these are found abundantly also in the mesonephros and even to 

 some extent in the heart. 



Must the umbilical vesicle then be regarded simply as a rudi- 

 mentary structure "morphologically significant, but functionally nil" 

 (Selenka)? The results of the present study support this generali- 

 zation in all points but one, viz. the hematogeneous function. Indeed, 

 embryologists have always attributed to it the latter function in some 

 degree. The microscopic findings indicate with clearness the origin 

 of "hematogonia" (Maximow) from the mesenchyme of the umbilical 

 vesicle. The vesicle seems to function as the main source of the pro- 

 genitors of the fetal blood cells. Its peculiar structure resulting 

 from the folding and evaginations of its entodermal lining may be 

 due to a hereditary proliferative tendency of this layer consequent 

 upon its phylogenetic history. 



Material and Methods. 

 The material of this investigation consists principally of the um- 

 bilical vesicle of a 13 mm human embryo for which I am indebted 



