STUDIES IN MAMMALIAN EMBRYOLOGY. 298 
velop into the future yolk-sac) have all the same character, are 
equally thick and none of them flattened out. A flattening is 
distinctly noticed in fig. 26, where the hypoblast-sac is applied 
against the epiblast, and it is there at the same time noticed 
that the hypoblast cells adjacent to the polar knob (the future 
germinal area) do not take part in this flattening process. In 
other words, those hypoblast cells that will become the hypo- 
blast of the germinal area and of the embryo are already at this 
early stage histologically distinct from those that will clothe 
the cavity of the yolk-sac. This same differentiation can be 
noticed in the figs. 8 and 27, 14—20, and 39. 
For the appreciation of the facts here developed it deserves 
very special attention that the three embryos, from which the 
figs. 22—24, 25 and 26 were made, have been taken from the 
same uterus, and have undergone exactly similar and simulta- 
neous treatment. It may be called a fortunate chance by 
which a clearer insight is certainly gained into the facts here 
recorded, that these three embryos have attained slightly 
different stages of development, fig. 26 being in advance of fig. 
25, and this again being further developed than figs. 22—24. 
Compared with the stage of fig. 21 (which was obtained from 
a different mother), they appear to me to form a continuous 
chain, from which no other conclusions with respect to the 
development of the hypoblast can be drawn, than those which 
I have here brought forward.! 
If we want to know whether the different mode of develop- 
ment of the hypoblast in the hedgehog, as compared to its 
development in the rabbit, the opossum, the mole, &c., admits of 
any reasonable explanation, I think we may safely answer in the 
' The peculiar development of the hypoblast in the hedgehog was described 
by me at the Wiirzburg Congress (‘ Anat. Anz.,’ iii, p. 511). E. van Bene- 
den expressed doubts as to the validity of my interpretations (loc. cit., p. 514). 
Later on I obtained still more conclusive preparations in favour of the 
views held by me, and have given a summary exposition of the case, in the 
‘Anat. Anz.,’ iii, p. 906. This allows me to omit a further debate on the 
question, whether artificial retraction of the hypoblast might have brought 
about the aspect of some of the preparations given by me. ‘This question is 
discussed loc. cit., p. 909. 
