SAEGENT: THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 187 



nicka's statement, that there are sometimes two or three parallel Reissner's 

 fibres. Tlie manner in which tliese axons unite to form a single trunk 

 varies with different species and is characteristic of them; it will later 

 come up for further treatment. In Amia the more usual method is for 

 the axons to unite singly with the main trunk. In later development 

 they become more closely consolidated with Reissner's fibre, appearing 

 as fibrils coming from it (Plate 4, Fig. 31). 



As the cell develops, the protoplasmic strands which connect it with 

 the wall of the cord (Plate 4, Figs. 27, 30) become metamorphosed into 

 dendrites, and at the same time numerous other branching dendrites 

 arise and, growing out through the fluid of the canal in various direc- 

 tions, penetrate the cord on all sides of the canal (Plate 4, Figs. 28, 29). 

 Often a large process resembling an axon runs caudad through the 

 canal. After passing into the tissue of the cord these dendrites could 

 be traced no further. The cells which make up the w^alls of the cord 

 at this early stage have no cell walls that can be demonstrated, and the 

 dendrites are lost in the apparently homogeneous protoplasm filling the 

 spaces between the large nuclei (Figs. 28, 29). 



The nucleus of these posterior canal-cells is always large and sharply 

 defined. Tlie chromatin is variously arranged, but usual)}', especially 

 in later stages, forms a more or less irregular network. As a rule a 

 large spherical nucleolus is present. The cytoplasm of the cells in their 

 early stages is somewhat diffuse and stains very lightly (Fig. 27, cl. can.2J-). 

 Later it becomes denser, and the cells take a more definite outline and 

 stain sharply (Fig. 28). 



By the sixth day of larval life tliese posterior canal-cells have in- 

 creased in size so that one of the larger of them, having a diameter of 

 8 to 10 micra, nearly fills the lumen of the canal. In Figure 28 

 (Plate 4) such a cell is shown lying in the ventriculus terminalis, 

 30 micra (three sections) from the end of the lumen and 60 micra from 

 the extreme end of the cord. Another such cell, lying just anterior to 

 it, and distant about 30 micra, is shown in Figure 29. These multi- 

 polar cells send their processes pei-ipherad about equally in all 

 directions. 



During the third and fourth day of larval life the system of axons of 

 the posterior canal-cells growing cephalad, now more or less consolidated, 

 meets, in the posterior third of the canal, the system of axons from the 

 tectal cells growing caudad, and by some process of interlacing or inter- 

 calating not yet clearly made out, the two systems unite to form the 

 so-called Reissner's fibre (Plate 4, Fig. 31, fhr. Meis. ; Fig. J7). 



