410 A. A. W. HUBRECHT. 
tion out of a complex of cells which is comparable to the mono- 
dermic blastula. And similarly in Sauropsids and Elasmo- 
branchs delamination is the process by which entoderm and 
ectoderm are respectively differentiated from one another. 
This takes place in a horizontal plane in consequence of the 
accumulation of yolk substance. In Amphibia, Cyclostomes, 
many Ganoids and Dipnoi, which have been looked upon as 
important supports for the view that gastrulation is effected 
by invagination, it has appeared more and more evident to 
successive investigators (Bellonci, Graham Kerr, Brauer, 
etc.) that here, too, the primary separation between ectoderm 
and entoderm is brought about by a process of delamination. 
As soon as the so-called blastopore (Rusconian anus) appears, 
which travels a certain distance over the surface of the egg, 
and which, in many cases, is turned into the definite anus, we 
have no longer before us the gastrulation process, but a pro- 
cess by which the metamerical and bilaterally symmetrical 
dorsal organs and the notochord are budded into existence. 
Thus the naked facts force the conclusion upon us that in 
the Acrania (Amphioxus) the gastrula arises by invagination, 
in the Craniata (all other Vertebrates) by delamination. Once 
the didermic gastrula-stage reached, a second phase of onto- 
genetic development is inaugurated which is also of high 
phylogenetic importance. In this phase the bilaterally sym- 
metric metameric animal gradually appears which we have to 
compare with possible phylogenetic transition forms that have 
connected the Vertebrates with radially symmetrical ancestors. 
This attempt at a plausible and rational reconstruction of the 
Vertebrate ancestry is, of course, hampered by the circumstance 
that no trace of those forms is any longer in existence. Still, an 
actinia-like, vermiform being, elongated in the direction of 
the mouth-slit, imposes itself upon our imagination, such as 
has served for the theoretical speculations of Sedgwick on 
this same subject, and has once been accepted by van Beneden 
for the precursors of the Chordata. 
We have already above considered that the processes of 
growth by which a Ccelenterate gastrula becomes fixed and 
