58 



EDWARD MCCEADY, JR. 



trophoblast thickens again, and many of Hartman's own 

 figures ('19, pi. 22, figs. 1, 2, 4, 4A, 3B, 5, 6, 8, 9) show 

 practically no differentiation between trophoblast and 'em- 

 bryonic area' except that of staining 1 reaction. In his figures 

 2 and 4 the trophoblast at the opposite pole is decidedly 

 thicker than the 'embryonic area.' Selenka (Taf. XIX, fig. 6) 

 shows the same thing. In still later stages (stages 20, 21, and 

 thereafter) the trophoblast at the opposite pole becomes at 

 least twice as thick as the medullary plate, but the medullary 

 plate is thicker than the immediately adjoining non-medullary 

 ectoderm and much more dense than the opposite trophoblast. 



Fig. 18 Stage 22. A, photograph of whole mount 16139. B, photograph of 

 section of same specimen showing medullary plate, notochordal plate, parietal 

 mesoderm, and shell membrane. C, more lateral view of same section. 



There is thus no stage after its first appearance when this 

 ectodermal plate is not sharply demarcated in sectioned speci- 

 mens by density, staining reaction, thickness, or all three, 

 from the rest of the vesicle. In external views (see the second 

 normal stage plate) it is easy to recognize until stage 22. 



Figure 18 A is a photograph of specimen 16139 (stage 22) 

 before it was sectioned. The long transverse line indicates 

 the level at which the section represented in 18 B and 18 C 

 was taken. The two arrows above the line show the extent of 

 the section photographed in 18 B, and those below the line 

 show the extent of 18 C. Figure 18 B shows clearly that the 

 parietal mesoderm is lateral to the thick ectodermal plate. 



