ERYTHROPOIESIS IN YOLK-SAC OF PIG EMBRYO 291 



ucts. In the* light of the data from pig embryos my subse- 

 quent ('10) interpretation as secretory structures appears to 

 have been correct. The filaments have a basophilic staining 

 reaction, hence stain well in specific mucous dyes. In later 

 developmental (functional) stages they are limited to the basal 

 ends of the cells, where they may become clumped into a deep 

 staining irregularly oval mass. The 'mucinous masses' described 

 for the yolk-sac of 9 and 13 mm. human embryos are essentially 

 the same structure as the presecretion filaments of the 10 mm. 

 pig embryo ; and their functional role is most probably secretory. 



What then may be the meaning of the yolk-sac entoderm in 

 terms of function? The additional evidence from the yolk-sac 

 of the pig, further supports my earlier conclusion ('07) that 

 this cell structure is to be interpreted in terms of the ancestral 

 history of higher mammals. In the ancestors with yolk laden 

 eggs the entodermal cells undoubtedly had the function pri- 

 marily of elaborating a digestive fluid for the liquefaction and 

 assimilation of the yolk. In yolkless umbilical vesicles, the 

 entoderm apparently still develops and differentiates in accord 

 with an 'ancestral memory,' though it can perform no true 

 digestive function. The umbilical vesicle of the pig, as of man, 

 is in large part — that is, as concerns digestive significance — a 

 vestigial structure. But it has taken on a secondary function, 

 now apparently become of great importance, as an early, perhaps 

 original, center of haemopoiesis. 



The above discussion would seem to dispose of Paladino's 

 (20) suggestion that the yolk-sac entoderm of higher mammals 

 has a hepatic function. The form and structure of the two 

 classes of cells are indeed very closely similar (figs. 31 and 32), 

 but this need not necessarily imply identity of function. The 

 similarity is due more probably to the fact of common origin 

 from the primitive gut, and the further fact that both are func- 

 tionally active, and in a secretory manner. Nor need the pres- 

 ence of glycogen in both types of cells be interpreted in terms of 

 functional identity, since many types of cells of embryos contain 

 glycogen [Gage (8)]. 



