174 THE ANCESTRY OF VERTEBRATES 



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". 



The 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 agastrula. 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 geniigende Ursachen vor, welche uns 

 nothigen wurden, in diesen einfachen EinstUlpungsprocess 

 alles Mogliche hineinzugeheimnissen und Schwierigkeiten 

 zu suchen, wo in Wirklichkeit keine vorliegen. Wenngleich 

 der Gastrula von Amphioxus gewisse Eigenthumlichkeiten 

 anhaften, so sind dieselben doch nicht so weitgehend, dass 

 sie uns zwingen wurden, an einer Homologie dieses zwei- 

 schichtigen Keimes mit ahnlichen 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 

 caenogenetjc. As a consequence of the accumulation of 



