GASTRULATION OF THE VERTEBRATES. 403 
The Gastrulation of the Vertebrates.! 
By 
A. A. W. Hubrecht. 
On p. 945 of the first volume of his ‘Handbuch der 
Entwickelungslehre’ Oscar Hertwig refers (in a postscript 
to his theory of the germinal layers) to a theoretical view 
concerning the process of gastrulation in mammals (and also 
in vertebrates generally) which I have attempted to establish 
in my article on ‘Keimblattbildung und Furchung bei 
Tarsius spectrum’ (1902, Verhandel. Kon. Akad. v. 
Wetensch. Amsterdam). He points out that I have “con- 
siderably changed my views,” and that I have ‘now been 
induced to look upon matters in a way which differs con- 
siderably from what he (Hertwig) and many other embry- 
ologists are ready to uphold.” 
This paper is meant to establish that the “divergence” of 
views which Hertwig accentuates will, in all probability, be 
only a temporary one. An explanation, or rather a rational 
interpretation, of what we understand by gastrulation as 
it has been introduced into science by Haeckel and Ray 
1 Since this article was first published in German in the ‘ Anat. Anzeiger,’ 
vol, xxvi, p. 353, several montlis have elapsed, and papers have appeared in 
that same periodical by Assheton (vol. xxvii, p. 167), and by Brachet (vol. 
xxvii, p. 212) concerning the same subject and referring to the original 
article. I must refrain from entering into any discussion in this translation, 
but will do so on another occasion. I only wish to point out that Assheton 
has quite misunderstood my German version in so far as he believes that I 
hold the Vertebrate mouth to be in any way derived from the stomodzum of 
an Actinia-like animal. My article “Keimblattbildung bei Tarsius,” cited 
above, leaves no doubt on this head. 
