340 ABSTRACTS OF PAPERS. [VOL. II. 



XI. AN APPARATUS IN THE CENTRAL NERVOUS 

 SYSTEM OF VERTEBRATES FOR THE TRANS- 

 MISSION OF MOTOR REFLEXES ARISING 

 FROM OPTICAL STIMULI. 



PORTER EDWARD SARGENT. 



IN Antia, at about the time of hatching, there arises in the 

 anterior portion of the roof of the optic ventricle a group of 

 cells, eighty to one hundred in number, formed by the differen- 

 tiation of indifferent neuroblasts. During the first and second 

 days of larval life the axons develop from these cells as exceed- 

 ingly fine processes, growing directly toward and into the optic 

 ventricle. Early in the third clay the adjacent axons come 

 together in groups and coalesce at their tips, in their further 

 growth through the cerebro-spinal fluid appearing as a single 

 fibril. Later these fibrils coalesce with others similarly formed, 

 and in their growth posteriorly through the ventricles and 

 canalis centralis form what has been known as Reissner's fibre, 

 which is then a fibre tract made up of many axons closely 

 united and surrounded by a single medullary sheath. Through 

 the posterior portion of its course there come off from it fine 

 fibrils which pass through the canal obliquely backwards and 

 enter the tissue of the cord. 



In the first clay after hatching there may be found in the 

 extreme posterior end of the canalis centralis a number of 

 small cells, three to four micra in diameter, lying in the lumen 

 of the canal and ventriculus terminalis. Some eight to ten 

 of these cells persist and continue to develop. Increasing 

 rapidly in size, they become spindle-shaped and send their 

 axons cephalad through the canal. The axons are at first 

 separate, but later coalesce as they grow forward, and, 

 eventually meeting the system of axons from the cells of 

 the tectum growing posteriorly, the two interweave in a way 

 not yet clearly made out. 



The development of this apparatus in Amia is typical of 

 its development in all vertebrates, though in some groups 



