CONTRIBUTIONS TO THE EMBRYOLOGY OF NEMERTEA. 427 
secondary epiblast can clearly be traced, would immediately 
reveal such a process by respective changes in situation of 
parts, &c. 
The second process can of course not be as emphatically 
denied as the first, but as the larval fore-gut always has a 
certain size when it originates, and was never seen directly to 
spring from the epiblast (vide supra), it is a far more accept- 
able view that the increase in depth of this front portion is 
due to repeated subdivision of the hypoblast cells forming this 
region, than to a similar process in epiblast cells which have 
not with certainty been demonstrated as constituents of this 
region. 
At all events it is evident that anyone wishing to persist in 
maintaining that the larval fore-gut is indeed an epiblastic 
stomodeum, is obliged to acknowledge that its formation or 
invagination begins and is clearly appreciable even before the 
then inferred closure of the blastopore has commenced. This 
is evident from such preparations as the one referred to (1. c. 
(30), pl. i, fig. 8). The closure of the blastopore must then 
take place later on, after it has wandered higher up into the 
gastrula. I repeat that this explanation appears to me highly 
artificial. For myself I fully accept the other interpretation, 
viz. that the external opening, leading into the gut, even when 
it has become narrow and crescentic, is still the original and 
permanent blastopore, which later on becomes the mouth of 
the adult without even disappearing, and the two cavities of 
the two regions of the gut are then wholly equivalent portions 
of the archenteron ; they have only become separated by an 
internal constriction and afterwards follow their respective 
destination along different lines of development. 
We must now inquire into the further phases through which 
these two portions of the intestine have to pass before attaining 
their ultimate structure. The posterior portion or larval hind- 
gut becomes the mid- and hind-gut of the full-grown worm, 
i.e. that portion which is characterised by the paired cecal 
diverticula and which extends unaltered down to the anus. 
From the anterior portion or larval fore-gut, on the contrary, 
