25 



them indicates a secretory activity of the cells. Moreover, an amorph- 

 ous coagulum is present in the tubules as in the cavity of the sac. 

 This coagulum in the sac is most probably the remnant of a fluid content 

 elaborated by the entodermal tubules. This may represent a secretion 

 needed in the preparation of the nutritive yolk before absorption in 

 the sacs of ancestors whose eggs contained abundant deutoplasm. The 

 entodermal cells might also conceivably secrete some substance directly 

 into vessels which closely surround the tubules. 



Columns of entodermal cells have already been mentioned (Fig. 8). 

 These also are surrounded by blood-vessels. Contrary to Meyer's (33) 

 observation for older vesicles, in both of the vesicles here mentioned 

 the line of demarkation between entoderm and mesenchyme is distinct. 



Historical. 



Ch. Kobin (37) appears to have been the first (1861) to make 

 a microscopic study of the human umbilical vesicle. He describes 

 the entodermal cells as containing fat vacuoles and yellow granules 

 (glycogen?) and as having a polyhedral form and a diameter of from 

 17 to 29 microns. 



Tubules were first (1896) definitely described in a comparative 

 study by Spee (46). They had previously (1889) been seen by 

 TouRNEAUx (48) and probably also by His (17) and Keibel (25). 

 They have also been described (1899) for apes by Selenka (45). 

 Lewis (29) gives a splendid illustration of the yolk sac wall with 

 tubules of a 9,4 mm embryo. In 1904 Meyer (33) described 18 

 vesicles of embryos from 11 to 110 days old. All vesicles between 

 the middle of the second and the middle of the sixth week had some 

 or many tubules. In 1906 £ternod (12) described both hollow and 

 solid entodermal evaginations in the yolk sac of a 3 mm embryo. 

 The yolk sac of a 9,2 mm human embryo, previously described (24) 

 was very favorable for a study of these tubules. 



Discussion. 

 As to the significance and function of the tubules Meyer ven- 

 tures no suggestion. A hepatic function as suggested by Spee, 

 Paladino and Saxer appears untenable for reasons above stated. 

 Since there is no yolk they cannot perform the absorptive function 

 of the entodermal cells lining the sacs of animals with yolk-laden 

 eggs. As previously expressed, the tubular arrangement may be 

 simply the result of an attempt on the part of the enlarging layer 

 of entodermal cells to adjust itself to its narrow confines. This 



