ERYTHROPOIESIS IN YOLK-SAC OF PIG EMBRYO 279 



III. DESCRIPTIVE 



a. The entoderm 



It seems preferable to begin the description of the histology 

 of the yolk-sac with the entodermal constituent of its wall. 



In the 5 mm. stage the entodermal cells are cuboidal, and 

 arranged in a single layer; there is as yet no trace either of solid 

 or tubular evaginations into the enveloping mesenchyma. 



In the 10 mm. stage of development the lining cells are colum- 

 nar, the taller being about twice the length of the tallest cells 

 in the earlier stage ; they are still arranged in a single layer. 

 However, there is great variation in the form of the cells; the 

 predominating type of entodermal cell is columnar, but all 

 transitional forms appear from very low cuboidal to tall colum- 

 nar cells. At certain points the entoderm invaginates the 

 mesenchyma in the form of short cords and tubules. The 

 'tubules' are scarcely more than shallow folds, but recall the 

 larger branched tubules of the yolk-sac of human embryos of 

 this length (Meyer (18) ; Jordan (10).) The condition is probably 

 to be interpreted in terms of a mechanical adjustment on the 

 part of the entoderm to the exiguous confines delimited by the 

 enveloping mesenchyma, or it may perhaps be merely a shrink- 

 age phenomenon. 



At the 25 mm. stage of developnient the entodermal cells 

 appear shorter columnar but are still almost invariably arranged 

 in only a single layer. Occasional small stratified areas occur 

 similar to those characteristic of the human yolk-sac of even 

 much earlier stages, but they are perhaps most correctly inter- 

 preted as short .stout entodermal buds or cords. At this stage 

 the very sparse enveloping mesenchyma is extensively invaded 

 by very numerous robust solid cords and irregular tubules of 

 entodermal cells. In tangential sections the yolk-sac wall of 

 this stage looks strikingly like reptilian liver tissue. 



The cytology of the entoderm is essentially identical for the 

 5 to 25 mm. stages (figs. 2 and 31). The vesicular nucleus is 

 relatively large and spherical, and is generally placed nearer 

 the basal pole. It contains one or several large, spheroidal. 



