125 
and, as explained in detail in the careful work of WILL 
(Zool. Jahrb. Vol. 6) (who in this confirms the ideas of 
BALFOUR, 1875), the bulky ventral wall of the archenteron 
can no longer be folded in, and the persistent invagination 
of the yolkless dorsal wall has the appearance of an 
independent ingrowth of the ectoderm”. 
he fact that in the blastula-stage the future ecto- and 
endoderm are often already recognizable by their cytological 
character may not induce us to deny that we have to 
deal with a blastula. Neither from the circumstance 
that in the gastrula-stage the future mesoderm cells may 
be already distinguished among the primary endoderm 
cells may we conclude that this stage is not a gastrula. As 
KORSCHELT and HEIDER (1910, p. 419-420) rightly remark 
concerning the gastrula of Amphioxus, the fact that the 
cells of the archenteron roof are a little (and how 
little!) smaller than those of the floor does not entitle 
us in the least to postulate a fundamental difference 
between Vertebrates and Invertebrates in the gastrulation. 
“Es liegen nicht genügende Ursachen vor, welche uns 
nöthigen würden, in diesen einfachen Einstülpungsprocess 
alles Mögliche hineinzugeheimnissen und Schwierigkeiten 
zu suchen, wo in Wirklichkeit keine vorliegen. Wenngleich 
der Gastrula von Amphioxus gewisse Eigenthümlichkeiten 
anhaften, so sind dieselben doch nicht so weitgehend, dass 
sie uns zwingen würden, an einer Homologie dieses zwei- 
schichtigen Keimes mit ähnlichen durch Invagination 
entstandenen Gastrulaformen vieler wirbellosen Thiere zu 
zweifeln.” 
HUBRECHT's speculations. — Yet particularly those authors 
who have occupied themselves principally with the embryo- 
logy of higher Vertebrates, viz: the Amniotes, have tried again 
„and again to take the gastrulation process in the latter as a 
base for their conception of the gastrulation of Vertebrates. 
Reading “the book of Nature upside down” (MACBRIDE, 
1909) they tried to explain from this aspect the simpler 
Processes in Anamnia and Acrania. Thus, as we have 
seen above, HUBRECHT and KEIBEL (1900) originally disting- 
uished two phases in the gastrulation, one in which, by 
delamination, the endoderm is formed, and one in which 
the rudiment of the notochord and mesoderm invaginates. 
The latter process is the palingenetic one, the former is 
caenogenetic. As a consequence of the accumulation of 
