19 



variatiou in thickness is due chiefly to the presence of tubules and 

 cords of entodermal cells. In the thicker regions also several layers 

 of entodermal cells are superposed. 



The lining entoderm is composed typically of large columnar or 

 pyramidal cells (Figs. 4, 5 and 6). The nucleus is large and centrally 

 placed. It contains one or several nucleoli and a reticulum with 

 numerous net-knots. The cytoplasm is coarsely reticular and contains 

 many large vacuoles and amorphous flakes and granules. Frequently 

 a cell is seen with two nuclei. The typical single-layered columnar 

 epithelium is seen only in the younger vesicle, and here only in those 

 regions where the wall is thinnest (Figs. 5 and 6). In other regions 

 the cells are crowded, superposed and of polyhedral shape. In the 

 older vesicle no region presents the typical single layer of columnar 

 cells. Where the wall is thinnest the entodermal cells form a single 

 layer but are much flattened and elongated. They contain small round 

 or oval pale-staining nuclei. The cytoplasm is finely granular or 

 'homogeneous and stains deeply. Frequently the cells are broken 

 and in general show signs of degeneration. Elsewhere the entodermal 

 lining may be described as a stratified polyhedral epithelium (Fig. 2 b). 

 The appearance is much that of the liver cells of amphibia or reptiles 

 except that the cells are larger in the wall of the sac. 



The nuclei appear perfectly healthy and generally contain numerous 

 large nucleoli. The cytoplasm, however, is extremely vacuolated and 

 contains numerous large deeply staining and variously shaped flakes 

 of doubtful nature. They may correspond to the mucinous masses 

 described in the younger sac, or they may possibly be flakes of glycogen, 

 or more probably, as Meyer (33) suggests, simply cell detritus 

 (Figs. 8 and 9). Sometimes the nucleus appears suspended within the 

 almost empty cell by delicate threads of spongioplasm. The cytoplasm 

 has the appearance of having sufi'ered degeneration. This is more 

 noticeable in the older vesicle. Judging solely from the appearance of 

 the entoderm in the older vesicle, one might be induced to conclude 

 that the vesicle showed signs of post-mortem disintegration or fixation 

 artefacts. But the fact that the mesenchyme and the blood cells are 

 perfectly normal and healthy, especially in the case of the 13 mm 

 embryo, leaves no doubt that the appearance of the entoderm is normal. 

 What degeneration has taken place is the result of a natural ante- 

 mortem process. Since it has proceeded farther in the older vesicle 

 it indicates that the function of the entoderm, whatever this may 

 have been, has passed its maximum of activity. The character of 

 the cells seems to indicate a secretory function, but no purpose for 



