STUDIES IN MAMMALIAN EMBRYOLOGY. 371 
In the latter part of pregnancy the whole ectoderm of the 
chorion has acquired the character of the cellular layer, except 
close to the margin of the placenta. . . . . The proto- 
plasmic layer may undergo complete or partial degeneration 
into canalised fibrine. . . . . The cellular layer remains 
on the villi only in a few patches and over the tips of certain 
villi, the protoplasmic layer of the villi remains everywhere, 
and develops numerous nodular thickenings.” 
The differentiation of the blastocystic ectoderm (our tropho- 
blast) into a tissue of varied aspect and function in man is thus 
more and more definitely established. At all events, it is 
difficult to abstain from attempting to draw comparisons 
between what obtains in man and what I have above described 
for a typical representative of the Insectivora. The remnants 
of an epithelial covering of the villi are—in contradiction 
with Turner—declared by Minot to be of embryonic origin, 
and a glance at our Pl. XXVI, figs. 55 and 56, will show that 
in this respect the facts that are presented by Erinaceus are all 
the more tempting for instituting a full comparative inquiry. 
At the same time, a comparison of Minot’s cut 35 (I. ¢., p. 424) 
of a normal human placenta of seven months with fig. 57 of 
this memoir, in which the ripe hedgehog’s placenta is repre- 
sented, is very suggestive. With respect to another passage 
in Minot’s summary, viz. “‘ No satisfactory explanation of the 
origin of the amnion has yet been offered. The placenta is an 
organ of the chorion; its evolution cannot be traced to modifi- 
cations of either the allantois or the yolk-sac,” I will have 
occasion to make certain remarks in the next chapter. 
V. Additional Reflections. 
There are certain points in the embryology of man and of 
mammals in general which appear to me to receive new and 
unexpected light from facts which have now been observed in 
the hedgehog. It would have led to confusion if I had referred to 
them while discussing those points in the development where they 
first presented themselves to my own mind. I wish to present 
