294 



embryo. Graf Spee and Meyer presumably, likewise, find no such 

 hyaline substance in their specimens, for they make no mention of it 

 in their descriptions ; this would seem to confirm Paladino's suspicion 

 that the "superimposed strata of transparent hyahne substance" were 

 due to post-mortem changes. 



The cells of the mesothelial layer vary, both as regards their 

 general form and the shape of the nucleus, according to different lo- 

 cations (Figs. 3, 4, 5). These variations in form are due undoubtedly 

 to variations in tension resulting from a greater or lesser distension 

 of the vessels and glands in the middle layer. In all cases the cells 

 are smaller, as are also the nuclei, than those of the entodermal layer ; 



Fig. 1. A 9,2 mm human embryo showing the yolk sac attached by the 

 pedicle. The amnion has been cut open and partially removed. Photograph, ^ 2'/3. 



moreover, the nuclei are more highly chromatic and are situated 'in 

 the proximal portion of the cell and only rarely can nucleoli be de- 

 monstrated. This zone is composed everywhere of a single layer of 

 cells. The cells vary in type from short columnar to squamous; both 

 of these types are rare, however, the flattened cell type predominating 

 (Figs. 3, 4, 5). 



The middle zone layer is the widest of the three. Histologically 

 it is a layer of connective tissue of the mucous embryonic type. To- 

 gether with the outer zone, it forms the splanchnopleuric layer of the 



