STUDIES IN MAMMALIAN EMBRYOLOGY. 507 



so that, even if the question were to be answered in the 

 affirmative, we should have to add that the possibility of distin- 

 guishing potential epiblast- from potential hypoblast-cells is 

 only limited to that very early stage. 



When once hypoblast-cells have begun to emigrate from the 

 embryonic knob towards the periphery of the blastocyst a stage 

 is soon reached in which, in the region of the embryonic knob^ an 

 outer layer — more than one cell thick — of epiblast can be dis- 

 tinguished from a subjacent stratum, of which the cells have 

 assumed a more flattened aspect (fig. 24'), and are continuous 

 with the hypoblast-cells beyond the embryonic knob. From 

 this moment onwards we shall do well to drop the term of em- 

 bryonic knob, and to call the thickened circular or oval patch 

 of embryonic epiblast, in accordance with the name chosen by 

 Bonnet and other authors, the embryonic shield. This is thus 

 a purely epiblastic structure, whereas the embryonic knob con- 

 tained both epiblastic and hypoblastic elements. 



As the blastocyst gradually enlarges, and the zona still 

 further attenuates, the embryonic shield increases both in size 

 and in thickness (figs. 12 and 25 ; 26 and 31). The hypoblast 

 now forms a complete and closed sac, clothing the entire 

 inner surface oft he trophoblast. This completion of the 

 hypoblast into a closed and independent sac (nowhere coales- 

 cent or fused with the epiblast) is thus attained before the first 

 trace of the origin of a middle layer has become apparent. The 

 actual diameter of the blastocyst is now about 08 to 1 mm. 

 Reconstructions of the entire surface of the embryonic shield 

 out of a continuous series of sections show that in this stage 

 the shield is hardly ever quite circular, but has generally an 

 unmistakable oblong, sometimes an ovoid, shape, the thinner 

 end of which corresponds to what will afterwards be the ante- 

 rior, the thicker to the posterior end of the embryo.^ 



^ In some specimens (fig. 21) it is the anterior end that is the broader. 

 Even in the later stages of PI. XXXVIII the oblong shield is sometimes wider 

 posteriorly, though generally anteriorly. At the same time it must not 

 be forgotten that the surface views of figs. IG — 21, 32 — 35, 62 — 64, and 

 79 — SI were not drawn from the fresh specimens, but are careful recon- 



