302 



month. Neither a glycogenic nor a haematogenous function of the 

 yolk sac thus relates it more closely to the liver than to several other 

 embryonic organs. If the yolk sac by virtue of its glandular structure 

 performs any "hepatic function" it is rudimentary and probably always 

 unnecessary for by the time that the embryo has developed eighteen 

 (18) somites (the period recorded by Graf Spee for the first appearance 

 of the glands) it already has a v?ell developed liver. 



A possible explanation of the gland formation may be found in 

 the fact that extensive development of the entodermal lining and 

 vigorous activity of its cells is simply the survival of an hereditary 

 force. Where yolk is plentiful as in the sauropsida and the egg-laying 

 mammals (Ornithorhynchus and Echidna) the function of the entoderm 

 is to absorb the yolk and elaborate it into food material to be carried 

 to the embryo by the vitelline veins. In man, where yolk is absent, 

 the hereditary power inherent in the cell forces the entoderm to a 

 development out of all proportion to that of the overlying and, in 

 this case, confining mesoderm. In consequence it becomes a mechanical 

 necessity for the entodermal layer to evaginate and fold into crypt- 

 like structures, the mesoderm folding in over these, or, where this is 

 impossible, becomes attenuated as is evidenced in places by the flattened 

 character of the mesothelial cells. 



My own observations lead me to the conclusion that the sole 

 function that the human yolk sac appears to possess for the embryo 

 is that assumed by the mesoderm in the production of blood islands 

 and the resulting vascular system, thus supplying the first progenitors 

 of fetal blood cells. The complicated histological structure is probably 

 due largely to the phylogenetic development of the entoderm. 



Literature. 



1) Gage, S. H., Glycogen in a 56-day Human Embrj'o and in Pig 

 Embryos of 7 to 70 mm. Am. Journal of Anatomy, Vol. 5, 1906, 

 No. 2, Proc. Ass. Am. Anat, p. XIII— XV. 



2) HuBRECHT, A. A. W., The Placentation of Erinaceus Europaeus, with 

 remarks on the Phylogeny of the Placenta. Quart. Jour. Micr. Sc, 

 Vol. 30, 1889, p. 283—404. 



3) Lewis, F. T., Yolk Sac. Ref. Handbook med. Sciences, Vol. 8, 

 p. 334—336. 



4) Meyer, A. W., On the Structure of the Human Umbilical Vesicle. 

 Am. Jour, of Anat., Vol. 3, 1903, p. 155 — 166, 



5) OsBORN, H. P., Observations upon the Petal Membranes of the 

 Opossum and other Marsupials. Quart. Jour. Micr. Sei., Vol. 23, 

 1883, p. 473—484. — The Petal Membranes of Blarsupials. Jour. 

 Morph., Vol. 1, 1887, p. 373—382. 



