286 A. A. W. HUBREOHT. 
I. Early Stages in the Development of the Hedgehog 
and general sketch of the Development of Yolk- 
sac and Allantois. 
The youngest stage of the hedgehog’s blastocyst that was 
observed by me is figured on Pl. XV, figs. 4—6, and Pl. XVII, 
fig 21. Two embryos of this stage are available.' The uterus in 
which they were found probably contained more, but a portion 
of it was accidently lost. The blastocyst here figured measures 
about +, mm.; it has the form of an oblong sac, and lies—as 
will be more fully described in another paragraph—as yet free 
in a recess of the uterus lumen. Its wall is formed by cells that 
are here and there more than one layer thick. A more con- 
siderable multicellular prominence in this wall is noticed at 
that pole of the long axis of the blastocyst which is turned 
away from the mesometrium. Inside this outer wall there is 
a small aggregate of perhaps half a dozen cells, which will 
develop into the hypoblast, and which appear to adhere more 
closely to one of the lateral walls (as distinct from the upper 
multicellular wall) of the blastocyst. In what is approximately 
the median section I count about thirty-two cells in the cir- 
cumference. I could not determine from stages which I per- 
sonally examined how this earliest blastocyst arose during the 
segmentation process, nor what is the exact mode of origin of 
the hypoblast cells. Keibel has lately described (‘ Anat. 
Anzeiger,’ iii, p. 631) an earlier stage in the hedgehog’s de- 
velopment in which the number of cleavage cells is only t wo, and 
which for that reason does not throw any light on this question. 
Later researches will especially have to make out whether there 
is any agreement between the mode of origin of the hedgehog’s 
1 For the preservation of my embryos I have used both Kleinenberg’s and 
Flemming’s solution, but prefer the first. When it is not otherwise indicated 
the preparations here referred to have passed through the first-named reagent. 
For staining purposes I have found that Ranvier’s picrocarminate gives the 
most reliable and durable, if not also the most brilliant, results. Caldwell’s 
automatic microtome considerably facilitated the manufacture of the very 
numerous series of sections upon which this memoir is based, 
