SPOLIA NEMORIS. 109 
advanced. The segmenting ovum of mammals may thus be 
said to present the peculiarity, that among the products of the 
holoblastic segmentation only one or very few cells represent 
the real embryo; whereas a very considerable number that 
rapidly expand into a vesicle (against the inner wall of which 
the hypoblast becomes applied later on, either in one way or 
another), are segregated at an uncommonly early period in 
order that they may help to bring about a satisfactory attach- 
ment between the blastocyst (which sensu strictiori isas yet 
enclosed in this early vesicle) and the mother. 
Only when the inner cell-mass shows the first traces of 
differentiation between those elements that will become hypo- 
blast and those that will become epiblast cells, is the stage 
reached that corresponds to the blastula of Amphioxus; only 
then there can be question to look for the homologue of the 
segmentation cavity. As hypoblast and epiblast are at first 
firmly pressed together, this segmentation cavity is even then 
not yet present. The monodermic mammalian blastocyst is 
thus a pseudo-blastula stage, its cavity is not the real segmen- 
tation cavity, but a cavity which could not fail to arise ever 
since, for purposes of attachment and nutrition, an extreme case 
of precocious segregation of certain epiblast cells has come to 
occur in the mammalian ontogeny. These cells arrange them- 
selves into a vesicle even before the two primary germinal 
layers of the embryo have differentiated. 
There can be no doubt, however, that, phylogenetically, it is 
the epiblast from which these cells have been segregated ; and 
this explains the intimate fusion which, after a certain time, 
obtains between this outer layer and the embryonic epiblast at 
the periphery of the latter. 
If I am right in upholding that the cavity inside the mono- 
dermic mammalian blastocyst is not the segmentation cavity, 
and that this blastocyst is only a pseudo-blastula, then we 
must similarly conclude that it is not a real holoblastic 
segmentation which the mammalian ovum undergoes. Even 
the name of tertiary holoblastic, which Rabl proposes to 
apply to the mammalian ovum (‘Theorie des Mesoderms,” 
