504 A. A. W. BUBRECHT. 



to or enclosure by the uterine wall^ as was described for the 

 hedgehog (16). 



Of the stages following upon the one here noticed numerous 

 specimens are at my disposal, and as long as the blastocyst 

 remains in the raonodermic, and even in the didermic stage, it 

 is either quite free in the lumen, or it shows the very first 

 signs of bulging out, and adhering with its outer layer against 

 the uterine epithelium. However, it has first to undergo a 

 considerable increase in size, and during this growth there is 

 at first an evident stretching of the existing cells, much more 

 than an active subdivision and increase in number. Simulta- 

 neously with the stretching of the widening blastocyst the 

 zona pellucida diminishes in thickness (figs. 6 — 11, 22 — 24). 

 In the last phases of the didermic stage, just before the first 

 traces of a mesoblast and of a gastrula ridge appear, the zona 

 has reached its limit of tenuity (fig. 26). After that it dis- 

 appears, and the definite attachment of the trophoblast to the 

 uterine tissue is brought about. 



In the preserved specimens which I examined, this increase 

 in bulk of the blastocyst, coupled with the decrease in thick- 

 ness of the zona, causes the blastocyst to lose its spherical 

 shape, and to fall into folds of very varied form and dimension 

 (figs. 8—12, 22—27). This folding— which is no doubt an 

 artefact, as we may suppose the blastocysts to be spherical 

 and elastic in life — is only counteracted when the trophoblast 

 begins to adhere against the uterine walls (fig. 12). The 

 blastocyst is then seen to reassurae, in sections of uteri that 

 were preserved in toto, its spherical aspect. For our appre- 

 ciation of the developmental processes that go on in the 

 trophoblast and in the inner cell-mass — which might also be 

 designated as the embryonic knob — these folds are but of very 

 little consequence, and do not interfere with an accurate 

 interpretation of the phenomena. 



To prove that indeed the increase in size of the blastocyst 

 to its double diameter (i. e. an increase in bulk of eight times 

 the original cubic span, and a surface increase of one to four) 

 is more a phenomenon of stretching than of cell-division, we 



