STUDIES IN MAMMALIAN EMBRYOLOGY. 287 
hypoblast and what was described by Selenka (‘ Studien iib. 
Entwgesch.,’ Wiesbaden, 1887) for the opossum. He mentions 
an inwandering of one or a few of the cells forming the blastula 
wall. These primordial hypoblast cells are large-sized ; they 
are first found in the segmentation cavity in a very small 
number and then multiply. New cells at the same time 
budding off from the spot which Selenka, with apparent good 
reason, Calls the blastopore. Gradually the hypoblast clothes 
the inner wall of the blastula, which thus enters upon its 
gastrula stage. The whole process is clearly illustrated by 
Selenka’s figures (10 and 11, pl. xvii; 2 and 8, pl. xviii) of the 
actual sections. Wishing for the moment to abstain from com- 
parisons between different modes of development of the inner 
cell-mass, as described for the mole by Lieberkiihn and Heape; 
for the rabbit by van Beneden, Kolliker, and Hertwig, I must 
emphatically re-affirm that the facts observed by me in the 
hedgehog leave no doubt but that the inner group of cells 
represents the hypoblast only. How this embryonic layer 
takes its origin out of them can be ascertained in the next 
phases. The developmental stages following upon the one just 
described show an increase in size of the blastocyst and an 
increase in bulk of the inner (hypoblastic) cell-mass. There 
is the same polar thickening of the blastocystic wall, and all 
over the remaining surface there are indications that more 
than one layer of epiblast cells contribute to the formation of 
the wall. In figs. 22 and 238 the polar thickening is just com- 
mencing to protrude a little into the segmentation cavity ; in 
figs. 8, 10, 25, and 26 this protrusion is much more marked. In 
the stage figured on Pl. XXII, fig. 39, it is evident that at this 
period of the development the polar projection forms a distinct 
knob at the anti-mesometrical extremity of the blastocyst, and 
that at the same time the remaining walls of the blastocyst 
have reached their relative maximum of thickness. At this 
stage the cell material of the lateral walls is seen to be here 
and there interrupted by lacune, as if the growth of the whole 
mass did not keep pace with the increase in size of the blasto- 
cyst. Soon after their appearance these lacune in the outer 
