168 WALTER HEAPE. 
one larger and hyaline, the other smaller and containing a 
more dense vitelline material. The hyaline segment he calls 
the epiblastic, the more opaque segment the hypoblastic 
sphere. He then describes the order of the subsequent seg- 
mentation phenomena, and declares that the segments derived 
from the primary hyaline epiblastic sphere gradually grow 
round those formed from the primary hypoblastic sphere, and 
there results a structure precisely similar to that described 
above (p. 166), which he calls the ‘‘ metagastrula” stage. This 
metagastrula Beneden compares with the gastrula of lower 
types, and he derives the epiblast of the blastodermic vesicle 
and of the embryo from the outer “ epiblastic” spheres, and 
the hypoblast and a portion of the mesoblast from the inner 
“‘ hypoblastic spheres.” 
There can be little doubt however, that Beneden’s account 
of the derivation of the layers is incorrect, and that the greater 
portion of the inner segments, as well as the whole of the outer 
segments, give rise to epiblast. When this is considered, and 
when the probable homologies of the primitive streak are recol- 
lected, any comparison of the so-called “‘ metagastrula” of the 
Mammalian ovum with the gastrula of lower types is found to 
be impossible, and the significance of whatever differences may 
exist in the two primary segments is rendered unimportant. 
In the absence of any figures in Beneden’s paper I have been 
unable to compare the appearance of the segments he describes 
in the Rabbit’s ovum with those I have examined in the Mole, 
but I have myself examined segmenting ova of the Rabbit, and 
have isolated the segments the one from the other, in order the 
more clearly to compare them, and in no case have I been able 
to distinguish the slightest difference in the density or con- 
stitution of these segments. 
If my observations are correct, then, the differentiation of 
the segmentation spheres into two layers in the fully segmented 
ovum is not a primary differentiation such as Beneden discerns, 
but a secondary differentiation due to the peculiar circum- 
stances of nutrition and development attending the formation 
of the Mammalian embryo. 
