422 ‘ F. KEIBEL. 
cess of invagination should in no way be involved. My 
arguments have been ventilated before (I. c.,! p. 1109 and 
1110), and I had come to the conclusion that in formulating 
the phenomenon of gastrulation the final result should be 
kept in view, and the special mode in which it is brought 
about should not be considered. I would then define as 
follows: Gastrulation is the process by which the cells of the 
Metazoan embryo are differentiated into ecto- and entoderm. 
By entoderm should only be meant those cells that will form 
the gut.’ 
(3) Hubrecht is, in my opinion, also justified—and it is a 
logical consequence of the definition—in emphasising that the 
primitive streak of the Amniota cannot be homologous with 
the primitive mouth (Urmund) or with the border of the 
blastopore. The primitive streak is a formation which has 
close and important connections with the blastopore, but 
which may not be without reservation homologised with it. 
(4) The difference which Hubrecht establishes between 
kephalogenesis and notogenesis should decidedly be 
acknowledged. The publications of Kopsch® (1896) and 
Jablonowski * (1898), to which I have already referred in a 
similar sense in 1901 (1. c., p. 1055-1061) should here be kept 
in view, and it should be specially insisted upon that the 
formation of the primitive streak stands in intimate relation 
to the phenomenon of notogenesis, and should thus for this 
1 Keibel, F., “ Die Gastrulation und die Keimblattbildung der Wirbel- 
thiere,’ ‘Ergebnisse der Anatomie und Entwickelungsgeschichte,’ Bd. x, 
1900. Wiesbaden, 1901. 
2 Jn his article ‘“ On Growth-centres in Vertebrate Embryos,” ‘ Anat. Anz.,’ 
T. xxvii, p. 167, Assheton has misunderstood the sense of my definition of 
gastrulation, as will be found out by what I have said here. 1 do not think it 
necessary to treat the question with more detail in this paper. 
3 F. Kopsch, ‘‘ Experimentelle Untersuchungen tiber den Keimhautrand 
der Salmoniden,” ‘ Verhandl., Anat. Gesellsch.,’ 1898. 
4 Jablonowski, J., ‘‘ Ueber einige Vorgange in der Entwickelung des 
Salmonidenembryos nebst Bemerkungen tber ihre Bedeutung fir die 
Beurtheilung der Bildung des Wirbelthierkorpers,” ‘ Anat. Anzeiger,’ Bd. 14 
pp. 532-57. 
