THE GASTRULATION QUESTION. 423 
additional reason not be homologised unreasonably with the 
blastopore. 
(5) I agree with Hubrecht in this, that I look upon 
Amphioxus as a form which occupies a position far away 
from the direct line of descent of the Vertebrates. I would, 
however, for the present, reserve my opinion as to the advisa- 
bility of any comparison with Balanoglossus, notwithstanding 
the observations which Hubrecht mentions concerning the 
formation of the pericardium in embryos of Tarsius, Tupaja 
and Sciurus. 
(6) It is my opinion, as it is Hubrecht’s, that the facts 
which have up to now come to light concerning the develop- 
ment of Vertebrates can, without difficulty, be brought into 
line with Hubrecht’s interpretation of the gastrulation process. 
The way to effect this has already been indicated by the 
hypothesis of the gastrulation in two phases, simultaneously 
formulated by Hubrecht and by myself, which I have later 
worked out more fully, and which has since been fully 
accepted also by Oscar Hertwig. However, in future we 
will only be allowed to call gastrulation that which I 
have defined as the first phase of gastrulation. The so-called 
second phase of gastrulation may no longer be looked upon 
as such. It is the process of formation of notochord and 
mesoderm, and is peculiar to Vertebrates. It should be 
borne in mind that in many cases it may often prove difficult 
to draw a strict line between the two processes, because, as in 
so many other chapters of embryology, processes may overlap 
which have phylogenetically been originally separated. 
In conclusion, I can say that in all the important points I 
agree with Hubrecht. In detail, several points of difference 
may be noted. Thus I believe that in the gastrulation of 
Vertebrates a more considerable part is played by invagina- 
tion than Hubrecht would be willing to concede. I am also 
in doubt as yet concerning the comparability between the 
development of the pericardium in mammals with certain 
featuresin Balanoglossus. Still, these and other points about 
which we have not come to an entire agreement while inspect- 
VoL, 49, voL. 2,—NEW SERIES, 30 
