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the fact that no karyokinetic figures could be seen may be due to 

 imperfect fixation. 



What then is the significance of the human yolk sac as evidenced 

 by its complicated histological structure? That both morphologically 

 and physiologically it is simply an expression of hereditary tendency 

 seems to be the trend of opinion. As a storehouse of nutritive yolk 

 it has no importance. Through the intimate relation between ovum 

 and uterine mucosa and the early establishment of a chorionic circu- 

 lation the embryo receives from the mother all necessary nutriment. 

 But the presence of a profuse glandular system suggests some secretory 

 activity. The relation of the blood system to the glands and the 

 similarity between the polyhedral entodermal cells and liver cells 

 suggests a hepatic function. The structural similarity alone, however, 

 seems altogether too slight to warrant the assumption that both perform 

 similar functions. Paladino beheves that the presence of glycogen in 

 the cells of the tubules points to a hepatic function. Recent investi- 

 gations by Gage have demonstrated that glycogen is a constituent of 

 the majority of the cells of many embryonic tissues. Cartilage, epi- 

 dermis, skeletal muscle and several other tissues are shown to be 

 glycogenic organs at some stage in the development. The presence 

 of glycogen in the entodermal cells of the yolk sac has consequently 

 no bearing on their supposed hepatic character. 



Graf Spee describes large epithelioid giant cells ("Riesenzellen") 

 in the entodermal wall of the yolk sac and glands. He believes these 

 cells to be the progenitors of the red blood cells, and consequently 

 ascribes to the yolk sac a haematogenous function. He identifies 

 these giant cells with the „primäre Wanderzellen" demonstrated by 

 Saxer in the yolk sac, liver and pronephros of the pig, sheep and cat 

 and described as giving origin to all blood cells both red and colorless. 

 The fact that the progenitors of the blood cells are found in both the 

 liver and the yolk sac adds little weight to Graf Spee's opinion that 

 there exists a similarity of function between these organs, for it is 

 evident that, based upon the presence of giant cells, a similarity of 

 function could with equal reason be assumed between yolk sac and 

 pronephros. 



Though this embryo was of approximately the same age as one 

 described by Graf Spee and the specimens examined by Saxer (about 

 ten [10] ram in length) I have failed to find conclusive evidence of 

 giant cells in the yolk sac. Nor have I succeeded in demonstrating 

 such cells in the yolk sac of an embryo (very abnormal) of about one 



