No. 6.] AMERICAN MORPHOLOGICAL SOCIETY. 341 







there are considerable variations. In the Skate the cells are of 

 conspicuous size, and three to four hundred in number. They 

 are multipolar, sending processes to the ectal portion of the 

 tectum, where they come in direct contact with the endings of 

 the optic nerve fibres and give rise to two fibre tracts, one of 

 which passes posteriorly to the cerebellum, the other anteriorly 

 and out into the optic ventricle to form the Reissner's fibre. 

 In reptiles and birds the apparatus appears at a late stage and 

 is not fully established until just before hatching. 



In Pctromyzon this apparatus is not fully established until 

 the second month of larval life. The cells, about twelve in 

 number, form a well-marked nucleus. Reissner's fibre passes 

 through the optic ventricle, reenters the brain tissue, and again 

 emerges into the fourth ventricle. Thus Petrotnyzon furnishes 

 the connecting link between the condition in the Gnathostomes 

 and AmpJiioxus. In the latter the largest and most anterior of 

 the colossal dorsal cells lies across the central canal, is in direct 

 connection with the pigment spot, and sends its axon caudad 

 through the cord in the median plane just ventral to the canal. 

 This axon probably represents Reissner's fibre. 



There are many lines of evidence which lead me to assign 

 the function I have to this apparatus. 



1. The cells are in direct connection with the endings of 



o 



the optic nerve, and with the cerebellum. The axons pass by 

 the shortest path posteriorly through the central canal, and 

 probably out through the ventral roots to the musculature. 



2. When the fibre is cut in Sharks or Dogfish they evidence 

 an inability to respond quickly to optical stimuli. 



3. In the vertebrates of the cave fauna the apparatus degen- 

 erates as the eye degenerates. 



4. In no animal does the apparatus reach complete develop- 

 ment until just before the animal attains free life. 



5. In those animals which are sluggish at hatching (Petro- 

 inyzon, Amia), the apparatus is not fully developed until a con- 

 siderably later period. 



6. In those mammals which are born blind (Mouse, Kitten), 

 the apparatus is not fully established until about the time the 

 eyes become functional. 



