CONTRIBUTIONS TO THE EMBRYOLOGY OF NEMERTEA. 429 
When the definite cesophagus has entered into its secondary 
communication with the hind-gut, the upper portion of the 
cavity of the larval fore-gut has become separated from the 
lower portion that gives rise to the esophagus. This upper 
portion is, moreover, characterised in this stage by the disap- 
pearance of its central lumen. Laterally two lumina remain 
persistent, which are thus direct derivates of the primitive 
archenteron. Cell-proliferation may also be noticed around 
them, and from these arise the paired nephridial ducts. 
When we remember that it is only in recent years that 
the nephridial system of the adult Nemertea has been defi- 
nitely recognised, and that even now it is not always easily 
demonstrable in the adult, it will be understood how exceed- 
ingly difficult it is to trace this system in the early stages of 
which we are now treating; several times it was even im- 
possible to detect its presence. For this reason I must 
congratulate myself that more than one observation has cor- 
roborated the views expressed above about the development 
of the nephridian system. A cellular, closed vesicle was more 
than once noticed, lying in the immediate vicinity of, but 
separated from, the cesophagus and evidently having developed 
one of the same mother-tissue (I. c. (380), pl. v, figs. 73—75, 
82—86). 
The nephridia apparently remain during a long period in a 
more or less embryonic phase. Oudemans (28) has demon- 
strated that the number of excretory pores (the secondary 
paired openings by which the nephridial ducts communicate 
with the exterior) increases as the animal increases in size, and 
this may further tend to prove that in Lineus the nephridial 
system attains its full development only late. This must partly 
serve to explain why I often found it so difficult—even in older 
larvee—to distinguish the excretory apparatus. I nevertheless 
feel convinced that the phases of development of this system, 
as traced above, are in accordance with the actual facts, 
although it is especially on this head that I look forward with 
great avidity to further evidence. All other points in the 
ontogeny of Lineus recorded in this paper have been verified 
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