108 A. A. W. HUBRECHT. 
to the solution of a puzzle which the comparison of the holo- 
blastic ova of Mammalia and of the lower Vertebrates and of 
Amphioxus present to us as yet. For my own part I hold that 
the principal reason why so many divergent and conflicting 
views have been consecutively adhered to with respect to the 
early mammalian blastocyst is this, that the appearance of a 
cavity in a segmenting ovum has never left room for a doubt 
whether this cavity could be anything else than a segmenta- 
tion cavity which, as such, was proclaimed to be homologous 
with that of Amphioxus. This homology, I hold, does 
not exist. It should be remarked that both epiblast and 
hypoblast, that will build up the embryo, are in the early 
monodermic stages of the mammals contained inside 
this cavity; and that we have to expect the real segmenta- 
tion cavity to arise between epiblast and hypoblast, as also in 
mammals it actually does later on. 
If the space enclosed in the earlier monodermic stages is not 
the segmentation cavity, then there will be no more a priori 
difficulties to understand that a great portion of it is afterwards 
converted into the archenteron. In fact, as little difficulty as 
that a part of the cubic space inside the shell of a hen’s egg 
becomes converted into the chicken’s cerebral ventricles. 
A comparison with another point in the embryology of the | 
higher Vertebrates will show that the above conclusion about 
the cavity of the monodermic mammalian blastocyst is less 
hazarded than might appear at first sight. 
Suppose for a moment that the details of the development of 
the amniotic Vertebrates were absolutely unknown to us, and 
that we were only fully acquainted with that of the anamnia. 
And suppose then some embryologist to teach that the differ- 
ence between the development of these anamnia and the as yet 
unknown higher Vertebrates would, for instance, prove to be 
this, that the latter manage to become suspended for a time in 
their own body-cavity, he would be in some danger of provoking 
hilarity, if not worse. 
Still we find no difficulty in interpreting the latter pheno- 
menon, thanks to the gradual steps by which embryology has 
