GASTRULATION OF THE VERTEBRATES. 411 
gradually changes into a sessile Actinian can hardly be looked 
upon as protracted phases of gastrulation. This will be more 
difficult yet when the animal has already acquired a higher 
degree of complication than that of the Coelenterates, and 
swims about in the shape of a worm-like, lower chordate 
animal. We know of Polygordius and of other primitive 
worm types that to the radial, didermic larval stage—the 
Trochophora—another developmental phase succeeds, during 
which we observe proliferation in the anal region, leading to 
an increase in the distance between the anus and the apex of 
Fic. 4.—A yvermiform protochordate with earliest differentiation of head, 
trunk, notochord, and incipient metamerism. 
the metamerical worm, the latter budding off, so to say, from 
the radial trochophora. 
We find similar processes in the Vertebrates, but without a 
free trochophora larva, and to this latter radial and didermic 
primitive stage corresponds in the Craniata the rapidly passing 
earliest phase in which delamination calls forth two germinal 
layers. Both in Elasmobranchs and in mammals we notice 
that the cellular material which is present in those very 
earliest stages contributes especially—as it does in the trocho- 
phora—towards the formation of the anterior part, the head, 
and that, following upon this, a proliferation-process is in- 
