iV> PROCEEDINGS OF THE AMERICAN ACADEMY. 



acteristic forms, reserving the more detailed account of the investigations 

 for :i later and more extended article. 



In Amid, at about the time of batching, there is in the anterior por- 

 tion of the roof of the third ventricle a differentiation of the neuroblasts. 

 Some of them have already increased in size, have become more nearly 

 Bpherical and take the stain more deeply than the surrounding cells 

 (Figure 1). These enlarged cells are concentrated for the most part 

 in the median plane at the anterior end of the tectum opticum near the 

 posterior commissure. During the first stage of differentiation there are 

 from twenty to thirty of these cells, but by the development of indiffer- 

 ent neuroblasts they increase rapidly in number, so that by the end of 

 the fourth day after hatching there are from eighty to one hundred of 

 them. During the tirst day the tectum is only one cell thick. Each cell 

 baa a large nucleus — whose diameter is somewhat more than one half 

 that of the cell — with a single deeply staining nucleolus. 



During the first and second days the axons develop from the cells as 

 emely fine processes, which always appear on the side of the cell 

 nearest the ventricle, directly towards and into which they grow. Early 

 in the third <lav adjacent axons, now projecting into the ventricle, come 

 together in groups and coalesce at their tips (Figure 2), appearing as a 

 Bingle fibril in their further growth through the cerebro-spiual fluid. In 

 later stages these fibrils coalesce with others similarly formed, and in 

 their growth backwards through the fluid of the ventricles and canal 

 form what has been known as Keissner's fibre (PI. 2, Figure 8.) During 

 larval development the fibrils which make up Reissner's fibre coalesce 

 more and more, until by the eighth day there are only six or eight separate 

 divisions of the fibre in the ventricle, and later these are still further 

 reduced in number by further coalescence. The cells also give off den- 

 drite-, which run backward through the substance of the tectum. 



In the later development of the tectum opticum these cells undergo an 

 apparent migration, due to the development of intervening tissue, as a 

 result of which their relative positions are considerably changed; the 



cells bec< • more crowded together. During these changes in position 



the axis of the cell may rotate through an arc as great as 180°. It is an 



interesting fact that the eccentric nucleus always retains its position at 

 the Bideoi the cell opposite that from which the axon emerges. 



At about the time of hatching one may find in the extreme posterior 

 end of th<- canalis centralis i Figure 5) a number of small cells, three to 



four iniera in diameter, lying in the lumen of the canal and ventriculus 



