STUDIES IN MAMMALIAN EMBRYOLOGY. 295 
cells form a continuous layer from the beginning. However, 
also on this point we would not be justified in drawing con- 
clusions from one case. More numerous examples from other 
orders will only enable us to decide about the phylogenetic 
value of the phenomenon to which I have here called attention. 
In the preliminary notice above referred to (‘Anat. Anz.,’ 111, 
p- 911), I have further pointed out the influence which these 
facts might have on more general speculations about the 
germinal layers of the higher Vertebrata, such as are contained 
in van Beneden’s paper in the ‘ Anatomischer Anzeiger,’ lil, p. 
710. He there declares that the two primary layers of 
mammals and birds, as they are known since the time of 
Pander and Von Baer, should not be regarded as homologous 
with the epiblast and the hypoblast of Amphioxus. I will not 
in this memoir return to the discussion of that point, as was 
my original intention, but will rather put it off till a later 
occasion when the formation of the mesoblast in the hedge- 
hog will be entered upon. The fact that Van Beneden’s more 
detailed paper has not yet appeared (July, 1889) makes this 
all the more appropriate. 
The early stages of epiblast and hypoblast have now 
received due notice. Between the moment of the separation 
of the embryonic epiblast from the polar projection till the 
first appearance of the primitive streak and the mesobiast 
there lies a period of growth during which the diameter of the 
blastocyst increases to about five times its length, the cubic con- 
tents thus being about centupled (cf. figs. 15—20 on Pl. XVI). 
During this rapid growth the hypoblast flattens out more and 
more with the exception of that in the germinal area, which 
retains the more cubic aspect it already showed in the very 
early stages. The outer epiblastic layer also flattens out, and 
gradually assumes a villiferous aspect (Pl. XVI, figs. 15—20, 
20a). The spaces between the villi correspond to what in the 
stage of fig. 14, Pl. XVI, and of figs. 7 to 9, Pl. XV, and 
fig. 39 of Pl. XXII, are the intercellular lacune in the wall 
of the blasto-cyst, and are there marked sp. 
The peripheral portions of what we have called the polar 
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