WEYSSE. — BLASTODERMIC VESICLE OF SUS SCROFA. 305 



cells ; on the other hand, these " Deckzellen " show unmistakable 

 signs of disintegration. The embryos represented in part in Figs. 1, 3, 

 and 4, Plate I., have these cells in a better state of preservation than 

 most of the others figured here. Fig. 27, Plate IV., is from a 

 section through the extra-germinal area of the embryo of Fig. 3. 

 Here, at the left and the centre, we see two " Deckschicht " cells, 

 which are entirely characteristic, the boundaries rather indistinct, the 

 nuclei round and with very little chromatic substance, the whole cell 

 flattened, in the plane of the surface of the ectoderm ; these occur all 

 over the vesicle at about the same distance apart. At the right-hand 

 side of the figure I have shown several such cells in contact with one 

 another ; this is the only place in my material where I have found 

 this phenomenon ; it suggested a possible earlier condition of these 

 cells. The cell boundaries, however, are very indistinct ; the cyto- 

 plasm is scarcely stainable at all, while the contents of the nucleus 

 stain a nearly uniform light blue. I am inclined, then, to regard 

 these cells as belonging to a purely transitory layer, which may for a 

 time serve some protective or other function, and then disappears 

 by the disintegration of its elements as it gradually becomes of no 

 further use. Bonnet ('91) says that this layer disappears early in the 

 sheep and in the pig, and he finds no trace of it in the youngest 

 sheep embryo he has described (Bonnet, '84, Taf. IX. Figs. 2 and 3.) 

 It should be remarked that in my oldest embryos, e. g. Fig. 6, 

 Plate I., there is scarcely any evidence that a " Deckschicht " has 

 been present, — only here and there a small nucleus attached to the 

 surface of the ectoderm. 



The third theory mentioned is that advocated by Minot ('85 and 

 '89) and at the same time by Haddon ('85 and 87) and later by 

 Keibel ('87). This theory starts, like the others, with an outer layer, 

 and at one pole an inner attached mass, and assumes that the whole 

 outer layer is entoderm, while the inner mass differentiates into two 

 superposed layers, an outer, which becomes the true ectoderm when 

 the entoderm outside of it, as a Rauber's " Deckschicht," disappears, 

 and an inner, which becomes the entoderm of the germinal disk. Mi- 

 not ('89) further suggests a complete inversion of the layers for all pla- 

 cental Mammalia. Earlier stages than mine are necessary for a full 

 discussion of this question, but the three-layer condition which I have 

 found over the greater part of the blastodermic vesicles of the pig, 

 seems to me an insurmountable objection to this theory. If the primary 

 vesicle is of entoderm, and the ectoderm later grows around it to pro- 



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