Johnston, The Brain of Petromyzon. 45 



is a row of nine or ten organs. A little nearer the mid dorsal 

 line and separated From the cephaHc organ of the last row by a 

 double interval, a row of six organs continues forward to the 

 tip of the snout. Caudally from the dorso-caudal border of the 

 eye a row of nine organs extends backward and slightly upward 

 to near the level of the first gill slit. Several (four) more organs 

 extend caudally at the same level but at longer intervals. Just 

 above and between each two gill slits occur one or two organs. 

 Finally, near the mid dorsal line an irregular paired dorsal row 

 of nine or ten organs extends through the gill region up to the 

 level of the pineal gland. 



In surface view, under a dissecting lens these organs pre- 

 sent a very characteristic appearance. The whole organ is 

 somewhat oval in outline, the long axis being in the direction 

 of the row of organs, and is bounded by a distinct whitish or 

 opaque lip or ridge. This ridge is broad and thick at the two 

 sides and is very small at the two ends of the oval. From this 

 it results that the ridges overhang and cover the greater part of 

 the pit which appears in surface view as an elongated furrow or 

 slit, usually narrower in the middle. In sections it is seen that 

 the organ is raised up on a pronounced papilla caused by a 

 thickening of the subcutaneous tissue which invades the space 

 between the pigment and fibrous layers of the dermis. The 

 ridge or wall of the pit is composed of the outer layer of the 

 epidermis, which is very much denser here than elsewhere and 

 is free from gland cells. This ridge is continuous with the sup- 

 porting cells of the floor of the pit, and the sense cells form a 

 barrel-shaped or elongated organ in the floor. It sometimes 

 happens that two organs in a line are so closely set as to appear 

 continuous in cross section and in other cases the single organ 

 is much longer than broad, but in any single section the organs 

 have a distinct barrel shape. The central part of the floor of 

 the pit is slightly or considerably raised, so that the pit often 

 appears rather as a deep and narrow ditch. The sense cells 

 (which are stained an intense black in iron haematoxylin while 

 the supporting cells are a grey blue) are spindle-shaped with 

 pointed -ends converging at the surface. The peripheral end of 



