294, A. A. W. HUBREOHT. 
affirmative. Rabbit, opossum, and mole correspond in one 
respect with each other in which they all differ from the hedge- 
hog, viz. that in all of them the blastocyst develops freely in 
the general lumen of the uterus, whereas in the hedgehog, as 
we will more fully discuss in a later chapter, the youngest 
blastocyst is encapsuled in a fold of the uterine walls that 
closes over it. This fact leaves a much more restricted space 
for the hedgehog’s blastocyst to undergo its early developmental 
phases in than what is available for the three first-named 
genera, that have the whole uterus lumen to extend into. 
The consequence is that at corresponding stages of develop- 
ment the blastocysts of the three first-named genera 
are of much larger size than those of the hedgehog. 
They are not only visible to the naked eye, but may be removed 
from the uterus with the tip of the scalpel.} 
The cubic size of the hedgehog’s blastocyst being thus many 
hundred times less than that of the rabbit, &c., we cannot 
wonder that this difference in size is reflected in the process of 
development of the primary layers. The smaller the space is 
which the blastomeres occupy in these early stages, the more 
probably the phases of a true holoblastic segmentation might 
be expected to reappear. ‘The influence of the large-sized 
yolk-sac on the early phases of development would then cer- 
tainly have diminished to some extent. And, I believe, that 
on these grounds we are justified in attaching considerable 
value to the phenomena of development of the hypoblast in 
those Mammalia where the early stages of the blastocyst are 
thus reduced in size. The more so as there actually is more 
similarity between these phenomena, as I have described them 
in the hedgehog, and such as we notice in those lower 
vertebrates where no yolk is present, and where the hypoblast 
1 This fact has, in the beginning, been the cause of the involuntary 
destruction of early developmental stages of the hedgehog, which I searched 
for in the way that is indicated by van Beneden, Heape, and others; but 
which [ did not succeed in finding in the uterus lumen, at the same time 
unknowingly injuring the spots where they lay concealed! Experientia 
docet ! 
