EARLY ONTOGENETIC PHENOMENA IN MAMMALS, ria 
Returning to the hedgehog for finding a reasonable 
explanation of how the folds may have arisen, when the 
amnion was no longer formed as a closed cavity ab initio, 
we see that here and in the bat a phenomenon occurs that 
does throw light on this point. We see (Fig. 38) that when 
a certain stage of development has been reached the circular 
rim of attachment of the ectodermal shield to the trophoblast 
shows a tendency to travel upwards. I have formerly (’95.8, 
p- 25) ascribed this to a splitting in the deepest tropho- 
blastic layers. I now feel inclined—as I did in a yet earlier 
paper (’89)—to see the first step in this direction in a more 
direct co-operation of that rim of the embryonic ectoderm 
itself, which travels upwards along the surface offered to 
it by the massive blood-laden trophoblast (Hubrecht, ’89 
p. 374, Pl. 25, fig. 51). The annular zone of attachment 
thus becomes more and more restricted, and when finally 
it disappears a separation between amnion and_tropho- 
blast is at that same moment brought about. Whether 
mesoblast has occasion to extend itself in the region between 
these two is a question in no way of vital importance for the 
amniogenesis, as is also demonstrated by the peculiar way in 
which the amnion arises in Chameleon, a sauropsid which, in 
this respect, undoubtedly reveals primitive characters. As 
such we reckon the fact that the amnion has, on starting, 
no lining of mesoblast, and that it has the shape of a ring- 
fold (Figs. 75, 76) closmg about the middle by a uniform 
constriction not yet differentiated into head-fold, tail-fold, 
and side-folds. 
This amnion fold of Chamzleo has an outer plait of 
trophoblast, an inner plait of embryonic ectoderm, the two 
growing independently and passing into each other at the 
rim of the fold where—as in Sphenodon (see also Figs. 77 
and 78)—lay in a somewhat earlier stage the potential line 
of demarcation between embryonic ectoderm and trophoblast 
alluded to above. These cases of reptiles are thus connected 
with that of the hedgehog above noticed and with that of the 
bats so very clearly figured by Duval (’99, Figs. 96, 102, 117, 
