HARLY ONTOGENETIC PHENOMENA IN MAMMALS. 7 
obtain such a working hypothesis. Both Sauropsids and 
Mammalia are, omnium consensu, phylogenetically derived 
from very early Protetrapods that lived about the Carboni- 
ferous period or even earlier, and which, in their turn, had 
aquatic and fish-like progenitors. These early, to us un- 
known, fishes have sprung from vermiform predecessors of 
ceelenterate pedigree. 
A tendency to exchange the radial for a bilateral sym- 
metry and to separate the ccelom from the enteron must at 
one time have characterised certain ccoelenterate ancestral 
forms, as has already been advocated by Sedgewick (784) and 
by myself (’05) on earlier occasions. It is not straining the 
imagination to assume that in this line of descent closely- 
related forms may have developed, some with, others without 
a larval envelope, temporarily ensheathing the cellular ele- 
ments that will build up the embryo itself and thus fore- 
shadowing the separation among their later, vertebrate 
descendants of such with and such others without a tropho- 
blast. 
We find examples of this amongst the Nemerteau worms. 
In some of these the egg after segmentation develops straight 
away into the young worm, in others, which as far as the 
typical Nemertean characteristics go are very closely 
related, the cleavage results in a disposition of the embry- 
onic material into (a) the first lineaments of the embryo itself 
and (b) a cellular temporary envelope of these, which is either 
more closely applied to (Desor’s larva) or more distant 
from (Pilidium larva) the material that goes to build up the 
embryo. 
And though I in no way want to infer that it is among the 
Nemertea or Gephyrea that we would have to look for the 
ancestral forms of the Vertebrates (uor either amongst any 
of the Annelids known to us) still it is an instructive fact 
that among different classes of worms (Gephyrea should also 
here be mentioned, see Fig. 129) the larval envelopes above 
alluded to are encountered in some but are absent in others. 
VoL. 93, PART 1.—NEW SERIES. 2 
