141 
Interpretation of the eccentric closure of the blastopore. — 
The second process mentioned above, the rostrad-eccentrical 
cosure of the blastopore, is not found in the frog egg; 
on the contrary, the closure proceeds caudad-eccentrically. 
| see in this caudad-eccentrical closure a result of the inter- 
ference of the contraction of the blastoporic border with 
a backward movement of the blastopore, following directly 
from my theory on the homology of stomodaeum an 
epichordal neural tube in Annelids and Vertebrates. As 
a result of the strong elongation, which we must assume 
the stomodaeum of Annelids undergoes in order to be 
transformed into the medullary tube of the Vertebrates, the 
entrance to the stomach, the cardiac pore, into which the 
blastopore passes, must move backwards over the whole 
length of the body —it moves still further, as we shall 
see (formation of the tail) —to become the neurenteric 
canal (also representing the former blastopore), This 
backward movement is performed in Chordates in anticipa- 
tion of the formation of a tube and during the contraction 
of the blastopore border, thus interfering with the gastrulation 
process. In this way the final, narrowed, blastopore is 
carried back to the place where it was originally found in 
Protaxonia, viz: diametrically opposite the animal pole. 
Whether this caudad-eccentrical closure of the blastopore is 
performed by concrescence or not is here of no importance; 
as stated earlier, Il do not believe that concrescence, in 
Amphibians at least, occurs to any considerable extent, and 
if it plays a rôle in more yolk-laden eggs, evidently no 
Primary significance should be attributed to it. The medullary 
plate in stage e (fig. 36), just as the rudiment of the 
stomodaeum in fig. 36 b, suirounds as a crescent the 
anterior border of the blastopore. This conception is reached 
for Amphioxus, e. g. by KORSCHELT and HEIDER in the last 
edition of their “Lehrbuch” (1910, p. 435), and especially 
for Ascidians by VAN BENEDEN and JULIN (1884, 1887), 
CASTLE (1896) and CONKLIN (1905) in their cell- lineage 
investigations. VAN BENEDEN and JULIN (1887, p. 259) state 
that the rudiment of the medullary tube surrounds the 
blastopore, presenting “la forme d'un anneau ou plus 
exactement d'une bague chevalière, l'anneau élargi en une 
plaque en avant du blastopore se rétrécit progressivement 
Sur les côté's et se réduit à son minimum en arrière de 
cet orifice” In contradiction to CONKLIN they emphasize 
