STUDIES IN MAMMALIAN EMBRYOLOGY. 291 
size, and can now harbour the whole embryo. For the rabbit 
a similar process has been fully figured by v. Beneden and 
Julin (‘ Archives de Biologie,’ v, pl. xxiv). 
We must now give a rapid glance at what further happens 
with the inner cell-mass, which I have designated as the 
earliest stage of the hypoblast. I find, what I hold to be con- 
clusive evidence, that this mass of cells does not gradually 
spread out against the outer epiblast, as it has been described 
by van Beneden for the rabbit and bat, by Selenka for 
numerous other rodents and for the opossum, by Heape for 
the mole; but that it becomes converted into the hypoblastic 
cell-layer by a different process. Instead of spreading out into 
a flattened layer gradually covering an increasing surface, 
losing in thickness what it gains in extent, and finally, only 
later on, when the whole inner surface of the blastocyst has 
gradually been clothed by it, attaining the form of a hollow 
sphere (cf. the well-known diagrams of the rabbit in all text- 
books), the hedgehog’s hypoblastic cell-mass bulges out 
gradually, leaving a central cavity between the hypoblast cells 
about in the same way as a morula stage that increases in size 
comes to enclose a segmentation cavity, and becomes a 
blastula. 
It is of course important definitely to establish the fact that 
in certain orders of mammals a similar mode of origin of the 
hypoblastic sac indeed occurs, and I have very carefully 
examined all my preparations of those early stages to see 
whether they would allow of another interpretation, To this 
I must decidedly answer in the negative, and must now some- 
what more fully explain the different sections and preparations 
on which I base my opinions. 
In figs. 22 to 24 three sections through a very young 
blastocyst are figured. The inner mass of hypoblast cells is 
cut across in all three, the sections evidently passing from a 
tangential towards a median plane, what is further confirmed 
by the following sections (that are not here figured), and in 
which a change in the opposite direction can be noticed, the last 
section again assuming the character of the first (i.e. of fig. 
