EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 169 
and even the fossil amphibian, known only by its footprints, 
Chirotherium, is supposed to have possessed this distinctive 
character of the Primates. So here, again, we find an 
indication that in trying to bridge the distance between 
Primates and the lowest Tetrapods the road is rather via an 
amphibian. ancestor than via reptilian-insectivorean transi- 
tional stages. 
We now come to the final paragraph of this chapter, in 
which we have to give our attention to the continuation of 
the Vertebrate (Chordate) pedigree among the invertebrate 
phyla. 
I have on a former occasion (’05a) expressed myself on 
this subject, but will here once more restate my own views. 
The diagrammatic type for a representative of the verte- 
brate phylum would be a bilaterally symmetrical, segmented, 
ccelomate animal, with the gut below, the central nervous 
system above the axial notochord. Following Sedgwick in 
his proposal to derive this type from an elongated, actinian- 
like starting-point, I have constructed a diagrammatic figure, 
which I here reproduce (Fig. 160), and in which I suppose the 
circumoral nerve-ring of the actinian-like ancestor to have 
become the central nervous system, the stomodzum to have 
become the notochord, the ccelom to have arisen out of the 
peripheral parts of the coelenteron. The original actinian- 
like dorsal mouth-slit (itself a differentiation of what was the 
blastopore of the gastrula-larva) is only evanescently repro- 
duced during vertebrate development by the communication 
between outer world and enteron, which travels backwards 
and separates the two halves of the concrescent notochord. 
The anus and the anterior neuropore may be two remnants 
of this slit; the vertebrate mouth is a neoformation, as are 
the gill-slits and the ccelomopores. In how far openings in 
the lateral body-wall, by which in certain living actinians the 
ceelenteron communicates with the exterior, belong tc the 
same category as the openings just mentioned cannot for the 
present be definitely settled ; nor can we know how many 
ccelomic pouches were present. at the starting, nor how 
