STRUCTUEE OF THE EXCRETORY ORGANS OE AMPHIOXUS. 191 
all the more easily, since a powerful ciliary current works 
towards the external pore. Sections of such injected speci- 
mens show conclusively that not a single particle of ink has 
entered the nephridial canal, although the ink has penetrated 
into every chink of the coelom. 
But, it may be asked, if the facts are so plain and conclu- 
sive, hoAV is it that so keen-sighted and accm-ate an observer 
as Boveri has been deceived? Well, if it will not be con- 
sidered presumptuous on my part, I will attempt to explain 
how the mistake arose. ^ To begin with, the sections he 
examined were not appropriate!}'’ stained. The nuclei are 
clear, but the cytoplasm scarcely stained at all. In the 
majority of the sections which I had the opportunity of 
seeing the wall which closes the tips of the diverticula was 
very difficult to make out, though I could detect it on close 
e.xamination in a suitable light. I naturally turned with 
great interest to the section given on PI. 33, fig. 17, of the 
original memoir (1), and of which a ])hotograph is published 
in the ' Anatomischen Anzeiger’ (la). Anyone on first 
looking at this section might be led to believe in the exist- 
ence of a funnel. The appearance is extraordinarily decep- 
tive. But it is deceptive, and the deception is due to two 
things. First of all the nuclei are deeply stained, but the 
cytoplasm practically colourless and transparent; in the 
second place the section is thick. The figure given by Boveri 
is really an optical section of the preparation. The closing 
wall can, indeed, be seen, but only with the greatest diffi- 
culty. The misleading appearance of a funnel is due to the 
sudden cessation of the nuclei round the base of the soleno- 
cyte tubes; an appearance which is further heightened by 
the limit of the coclomic epithelium at the same spot (see 
p. 193, and text-figure). 
Jjet it not be thought that in insisting on the absence of an 
opening I am unduly influenced by a priori considerations 
* Soon after the publieatiou of liis paper (la) I wrote to Professor 
Boveri, who then very kindly sent me his preparations, and I gladly 
take this opportunity of thanking him for his courtesy. 
