STRUCTURE AND DEVELOPMENT OE REIS^NEr’s FIBRE. 45 
(e.g. Selachians and Myxinoids), \vliei-e the specialised 
ependymal epithelium also may extend onto the dorsal 
(posterior) surface of the posterior commissure. 
III. Eeissner’s Fibre and the Sdb-commissural Organ 
IN the Petromyzontidai. 
Petromyzon fluviatilis. 
Of this species I have specially prepared and studied 
seven series of sections of the brain cut in the usual three 
planes, while of the terminal part of the spinal cord I have 
five series cut sagittally. In addition to these I have also 
examined several series through the brain and through the 
tail region of this animal belonging to the collection of 
King’s College. 
Upon the brain of one and another of the several species 
of this family so much has been written (Ahlborn, ’83, on 
P. plaueri; Dendy, ’02 and ’07, on Geotria australis; 
Johnston, ’02, on Lampetra wilderi; Sargent, ’04 and 
Sterzi, ’07, on Petromyzon marinus, to mention only a 
few of the many modern writers who have dealt with the 
region of the brain with which I am here principally con- 
cerned), that there will be need for me to say very little 
concerning the gross anatomy of the brain of Petromyzon 
fluviatilis. 
S u b - c o m m i s s u r a 1 0 r of a n . 
o 
Although a very conspicuous structure, the sub-conimissimd 
organ has apparently been described in some detail in three 
species only, viz.: Ammocoetes (Peti-omy zon) sp. 
(Dendy, ’02), Geotria australis (Dendy, ’02, ’07), and 
Petromyzon marinus (Sargent, ’04).^ In these, it is 
* Johnston ('07) does not even mention this structure, and Stei’zi 
merely speaks of a highly developed epithelium lining the “ recessi 
post-ahenulari ” in P. marinus. This he supposes (as Edinger before 
him had done in the case of Scyllium) to be a secretory epithelium. 
