STRUCTURE AND DEVELOPAIENT OE REISSNER’s FIBRE. 87 
condition in the preparation of the material, or whether the 
material was hardened entire and the recoil was the result of 
some breakage of the fibre in life, Studnicka in his paper 
affords no clue. The condition recorded by Sargent was 
certainly produced artificially by his treatment of his material, 
and Ids description of the end of the fibre in adult Petro- 
myzoii thus becomes nothing more than an account of a 
tangle in tlie hinder part of the central canal, which would 
apply equally well to a tangle occurring at any point (cf. my 
figure, ’12, fig. 3). 
Whether his failure to observe its proper point of ending 
iu the sinus termiualis was due to his failure to trace the 
filuin terminale to its end, or, as I think more probable, 
because the actual extremity of the central canal was lost in 
the preparation of his material, it is not now possible to 
determine. 
Notwithstanding that his description thus betrays the fact 
that he himself had failed to discover the normal ending of 
the fibre in the adult sinus termiualis, he nevertheless 
disputes the correctness of Studnicka’s statement that the 
fibre emerges from the end of the central canal into the 
lymph chamber which encloses the end of the neural tube. 
For this altogether unwarrantable proceeding I can find no 
excuse. Even if Sargent had correctly observed the condi- 
tion of the end of the fibre iu the ammocoete, he was not 
justified in an assumption that such a condition must neces- 
sarily persist iu the adult. As a matter of fact, I believe he 
is also mistaken in his account of the larval condition. I 
have not examined, it is true, larval material of P. planeri, 
but iu a very complete series of ammocoetes of Ichthyo- 
myzon trideutatus, ranging from 12 mm. to 105 mm. in 
length, I find nothing to confirm the account given by 
Sargent of the condition of the posterior end of the fibre in 
larval Cyclostomes. 
The sinus termiualis, as already stated, is present in 
this species as a terminal dilatation of the central canal, even 
in my youngest ammocoetes. Behind it is blocked by a solid 
