184 bulletin: museum of comparative zoology. 



The union of the axons in larvae of six days and more occurs 

 relatively nearer the point where the axons emerge than in younger 

 specimens, the coalescence and consolidation becoming more complete 

 Avith the advance in development. Compare Figures 25 (Plate 4) and 

 35 (Plate 5). 



Xear the median plane the axons run straight into the ventricle, con- 

 verging, with gradual coalescence, toward the fibre. Such axons as 

 emerge into the ventricle further from the median plane run out a 

 little distance from the wall and roof of the ventricle, then turning at 

 right angles unite, converging toward the anterior end of the ventricle, 

 where the united bundles curve backward and join with those of the 

 median plane and of the opposite side to form Reissner's fibre. 



The cells at this stage are still conspicuous and of large size, compared 

 vith the surrounding cells, having a diameter of from 12 to 20 micra. 

 From the cell arise, in addition to the axon desci'ibed, a number of other 

 processes. These could be followed only a short distance in Amia at 

 this stage. Some of them, however, run posteriad toward the ectal 

 region of the optic tectum. 



In the eight-day Amia, Reissner's fibre has become still more con- 

 solidated at the anterior end, that is, the coalescence of the axons has 

 continued to progress anteriad (Plate 5, Figs. 32, 33, 34). In this 

 stage some of the axons coalesce before emerging into the ventricle. 

 This was shown by carefully counting the number of cells (71), and 

 then counting the separate axons at the point where they enter the 

 ventricle (55). This counting could be done with some degree of 

 accuracy, and showed that there had been union within the tectum. 

 Counting now the separate fibres crossing an imaginary curved surface 

 located in the ventricle, and everywhere 0.1 mm. from its roof and 

 walls, the number is 25, and still nearer the median plane of the ven- 

 tricle tlie number is reduced to six or seven. 



In the seventeen-day stage the constituent elements of Reissner's fibre 

 entering the ventricle have undergone further coalescence. The an- 

 terior end of the fibre, however, still has a dendritic form (Plate 5, 

 Fig. 35), owing to the many separate bundles of axons which enter the 

 ventricle. These unite into several main trunks, which run nearly 

 parallel through the anterior portion of the mesocoele before coalescing. 

 From the middle portion of the mesocoele, caudad, the union is com- 

 plete. In the thirty-day stage there ai'e only six or eight chief divisions 

 of the fibre in the anterior portion of the mesocoele, and in the adult 

 this number is still further reduced. 



