430 A. A. W. HUBREOHT. 
over and over again: for the development of the nephridia 
however, I can as yet only refer to a more restricted number of 
observations, all nevertheless, in accordance with each other. 
There remains for the present a considerable blank in our 
observations between the vesicular stage of the nephridium and 
that in which we find it in the adult worm. 
Moreover, I must for the present leave undecided whether 
the whole of the cell-material of the primitive fore-gut is used 
up in the formation of esophagus and nephridia, or whether 
a portion of it is resorbed or converted into amceboid mesoblast 
cells. 
When the mid- and hind-gut has for the second time 
entered into communication with the exterior by means of the 
newly-formed cesophagus, its cell wall shows a very marked 
difference from that of the latter (cf. 1. c. (30), pl. v, fig. 81, 
also pl. vi, fig. 65). In some preparations I find the epi- 
thelium high, in others much lower; in some there is a decided 
lumen of this part of the gut, in others not. I would feel in- 
clined to accept the view that not all the original hypoblast cells 
pass into this epithelium but that some remain lying in the 
cavity of the intestine, and are there digested as embryonic 
pabulum. An anus is not yet present in these stages, even when 
the cesophagus has already coalesced with the intestine. There 
is a median longitudinal infolding of the intestinal epithelium 
along the back of the animal, in the region where the probo- 
scidian sheath will by-and-by develope. This fold (fig. 14) is 
best seen in transverse sections (1. c. (30), pl. iv, figs. 64—67). 
1 Tt is only with the utmost reserve that I venture to point to this origin 
of the nephridia as paired outgrowths from part of the wall of the arch- 
enteron, as heing a process which undoubtedly offers certain points of resem- 
blance with the origin of a true enteroecel. This ontogenetic process must, 
however, be studied in further detail, and more should moreover be known 
about the nephridial cavities in the adult throughout the whole class of the 
Nemertea before we are justified in proclaiming homologies between these 
latter cavities and the enteroccel of other Metazoa. Oudemans’ researches 
(28) do not lend any support to such homologies ; rather the contrary. 
