EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 151 
their situation in relation to the rest of the embryo is con- 
cerned, with the trophoblast of mammals. In the Amphibia, 
Dipnoi, and Teleostomi, however, the layer does not in any 
way participate in the formation of an amnion or of a foetal 
envelope, nor does it remain at a distance from the developing 
embryo, protecting it in some way or other. Its significance 
as a transitory outer membrane is, however, undeniable, even 
when its participation in the formation of certain superficial, 
mostly larval, structuresis remembered. And we are forced to 
consider whether we should not, for that reason, be justified 
in saying that, together with mammals and Sauropsids, these 
vertebrates have a common descent from ancestors in whicha 
transitory larval envelope played a prominent part. We yet 
notice a similar occurrence in different classes of Vermes 
(Nemertea, Gephyrea) where certain groups have definite 
larval layers which are absent in others. 
In that case a second consideration is this: do the carti- 
laginous fishes stand apart in that respect, and what about 
the Cyclostomes and Amphioxus ? 
About the absence in the latter genus of anything like an 
outer larval layer there can be no reasonable doubt after the 
numerous investigations concerning its early development 
which we owe to such a considerable number of trained 
embryologists. As to the sharks and rays, we can be equally 
positive that none of those who have studied their embryology 
up to now have cited any fact which would support the notion 
that anything like the ‘“ Deckschicht” of Teleostomes, 
Dipnoans, or Amphibia is present in any of them. We have, 
of course, the example of the Sauropsida to make us rather 
careful concerning cases in which there is an apparent 
absence of a trophoblastic layer. But then in this case the 
difference on many other points of comparative anatomy as 
between the cartilaginous fishes and the higher vertebrates is 
so considerable (as has already been partly pointed out above 
on p. 82) that it seems advisable to leave it open that the 
Selachians may very well have descended from ancestors 
without an outer larval layer. 
