SAKGENT: THE OPTIC ERFLEX APPARATUS OF VERTEBRATES. 159 



(p. 119). This functional relation between axis-cylinders in contact, 

 as I shall later show, exists between the axons making up Keissner's 

 fibre. 



(4) Finally, Hamaker believes that the giant fibres, or some of them, 

 have to do with the transmission of motor reflexes arising from optical 

 stimuli. Hamaker ('98, pp. 114-115) says : "In Nereis I have fre- 

 quently noted a sudden longitudinal contraction where there was 

 apparently no stimulus except the passing of a shadow. . . . Now, if 

 the shadow cast by a predatory animal were to bring about this move- 

 ment, the mechanism would be of vital importance to the worm. Per- 

 haps the importance of the function and the great extent of the 

 movement brought about help to account for the large development of 

 the giant fibres." 



In the phylogeny of the optic reflex apparatus the cyclostomes form 

 the connecting link between Amphioxus and gnathostomes in still an- 

 other way. The small number of the tectal reflex cells in Petromyzon 

 is intermediate between the one-cell state in Amphioxus and the con- 

 dition in Amia, where there are nearly one hundred cells, and that in 

 selachians, where there may be from three to four hundred. It is 

 interesting here to note that this optic reflex apparatus has developed in 

 dii'ect proportion to the complexity and importance of the visual organs. 

 Possibly some of the giant cells of Amphioxus which lie at the caudal 

 end of the cord and send their axons cephalad are phylogenetically 

 related to the posterior canal cells of the craniota. 



I have been able to study the posterior portion of Reissner's fibre in 

 only one adult Petromyzon. In this case the posterior 5 cm. of 

 the tail was severed from the fresh animal ; after removing the skin and 

 muscles, the cartilaginous axis with the enclosed cord was fixed and de- 

 calcified in Flemming's fluid. The preparation w^as then cut into sagit- 

 tal sections, and stained with iron hematoxylin. At the posterior end 

 of the central canal, where it dilates to form the ventriculis terminalis, 

 lleissner's fibre is found wound upon itself so as to form a tangled mass 

 (Plate 1, Fig. 8). This portion of the fibre has an increased diameter, 

 and its outline is not so sharp and definite as where its course is straight 

 through the canal. For some distance immediately anterior to the 

 portion of the canal shown in Figure 8, the fibre was thrown into loose 

 coils or had an undulating course, and further forward the canal was 

 empty. 



Sanders ('94, p. 44) has described a similar condition in Myxine : 

 "Here it [Reissner's fibre] is particularly well developed, and has in 



VOL. XLV. — NO. 3 3 



