448 PROCEEDINGS OP THE AMERICAN ACADEMY. 



and come ihto close proximity to the Purkinje cells. The fibre tract 

 which ruus forward may be traced to the anterior end of the tectum, 

 where the fibres converge, apparently forming Reissner's fibre, which 

 emerges from the tectum into the ventricle at this point. Where Reiss- 

 ner's fibre leaves the brain tissue, the membrane covering of the roof of 

 the ventricle is continued in a cone-like projection surrounding the fibre 

 (Figure 9). Still another process given off from these cells passes peri- 

 pherad nearly to the outer surface of the tectum, where it comes directly 

 in contact with the endings of the optic nerve fibres. These cells, whose 

 axons form Reissner's fibre, are, then, in direct connection with the eye 

 through the optic nerve, and in turn are connected with the Purkinje 

 cells of the cerebellum, which are known to coordinate muscular move- 

 ments. The posterior canal cells in the selachians are of relatively large 

 size and about a dozen in number. 



In the Cyclostomata this apparatus is in a more primitive condition. In 

 Petromyzon marinus it begins to develop some time after hatching, and 

 is not fully established until the second month of larval life. In larvae 

 thirty days after hatching the cells form a small but well-marked group 

 lying in the roof of the rudimentary optic lobes in the median plane, and 

 occupying the whole thickness of the tectum (Figure 7). The axons 

 emerge directly into the optic ventricle and unite in groups to form 

 Reissner's fibre. These axons may continue backward to the fourth 

 ventricle in three or four separate divisions, but in later development 

 these coalesce more and more. At this stage the posterior canal cells are 

 already developed and are sending their axons cephalad through the 

 canal, but they have not yet united with the axons of the tectal cells 

 running caudad. 



In the adult Petromyzon Reissner's fibre passes through the canalis 

 centralis and the fourth ventricle, from which it enters the brain tissue of 

 the basal portion of the cerebrum and passing through this emerges into 

 the third ventricle. Here it breaks up into several trunks and continues 

 forward to the anterior portion of the ventricle, where after further 

 division it enters the tectum. 



The passage of Reissner's fibre for a portion of its course through the 

 nervous tissue in cyclostomes is especially interesting as throwing light 

 on a little understood structure in Amphioxus. Of the dorsal colossal 

 nerve cells in the anterior portion of the central nervous system of 

 Amphioxus, the largest and most anterior, designated by Rohde as the 

 " kolossale ganglien-zelle," is median and lies across the lumen of the 

 canalis centralis. The cell is in direct connection with the pigment spot. 



