426 A. A. W. HUBRECHT. 
gave probability to this hypothesis. In the first place, I was 
able to show that the cesophagus of the adult worm developed 
out of this stomodeum. Secondly, Metschnikoff describes 
in both his papers (14 and 25) on the development of 
Pilidium and Lineus, an anterior portion of the gut which 
entirely corresponds in situation and delineation with the 
cavity just described for Lineus. Metschnikoff looks upon 
this portion of the gut as decidedly an epiblastic stomodeum, 
and also notes that the cesophagus of the full grown animal 
developes out of it. 
Further investigations carried on with very numerous larve 
of the earlier stages, obliged me to change my original inter- 
pretation just now stated. In the first place, I never found 
a preparation in which the blastopore became closed in loco, 
i.e. on the level of the epiblast. The tissue separating the 
larval fore- and hind-gut was always situated further inwards. 
It thus appeared more probable that the archenteron sub- 
divided into two portions of which the posterior one became 
entirely shut off from the exterior. I indeed succeeded in 
finding sections (Il. c. (30), pl. 1, fig. 8) in which the com- 
mencement of such a separation and local narrowing was 
clearly seen. This preparation at the same time convinced me 
that the cells which will clothe the larval fore-gut are quite as 
evidently hypoblast cells as are those which build up the wall 
of the hind-gut, and that they are in no way of an epiblastic 
nature. This is the more important, as very soon a certain 
though slight amount of difference between the cells paving 
both cavities can be observed, since on this account a presumed 
epiblastic origin of one of them might seem all the more 
probable. 
Now, if the larval fore-gut were indeed an epiblastic in- 
growth we would have to picture to ourselves its increase in 
depth, either by a continued infolding at the mouth, or by a 
continued cell division, new epiblast cells being sent inwards 
to complete the ingrowth when once it had begun. 
That no active continued infolding takes place can be easily 
yerified by comparison of the sections which, when once the 
