106 A. A. W. HUBRECHT. 
v. Beneden and Julin for the bat, by Heape for the mole, by 
myself for the shrew, and by Selenka for the opossum. 
The extension of the hypoblast against the outer wall of the 
blastocyst is obtained in a different way in the hedgehog, as I 
have elsewhere described.! Instead of having to spread out 
against the inner surface of the wall of the blastocyst, the 
hypoblast of the hedgehog is from the beginning a solid knob 
which develops into a closed sac by distension. Further dis- 
tension goes parallel to further growth of the didermic blasto- 
cyst. 
The cause of the difference in development of the hypoblast 
is most probably the ever so much smaller size of the hedge- 
hog’s blastocyst when compared in corresponding phases with 
that of the rabbit, mole, &c. This is in its turn caused by the 
fact that the hedgehog’s blastocyst, instead of being located in 
the uterine lumen, becomes included at a very early stage in 
the midst of maternal proliferating tissue. (‘ Anat. Anz.,’ 11, 
p- 906.) 
In 1892 Dr. Arthur Robinson? published a paper in 
which, starting from what he finds in the mouse and the rat, 
which he has studied for himself, he looks upon the process of 
hypoblast formation in the rabbit and bat in quite a different 
light than has been done by former investigators. The 
process in the hedgehog is, according to his views, more 
directly comparable to what he finds in the mouse. He has 
based on his observations a series of far-reaching theoretical 
speculations that partly correspond to views propounded by 
Sedgwick Minot in 1885.3 Robinson concludes that in 
mammals it is not the hypoblast that spreads against the inner 
surface of the epiblastic wall of the blastocyst ; but that, on the 
contrary, the epiblast, at a quicker or slower rate, spreads over 
the outer surface of a hypoblastic vesicle, which, according to 
1 « Anat. Anzeiger,’ Bd. iii, pp. 511, 906; and ‘ Quart. Journ. Mier. Sci.,’ 
vol. xxx, p. 291. 
2 «Quart. Journ. Mier. Sci.,’ vol. xxxiii, p. 369. 
3 *Buck’s Reference Handbook of Med. Sciences,’ i, 528, 1885; and 
‘ American Naturalist,’ September, 1889; also ‘ Human Embryology,’ 1893, 
p. 107. 
