THE PINEAL SYSTEM OF FISHES 



215 



above descriptions it will be seen that the pineal system of Amia shows 

 definite evidence of bilaterality, and also indications of degenerative or 

 devolutionary changes in both organs, but more especially of the left, 

 which in some cases exists only in the early embryonic stages of develop- 

 ment. 



Eycleshymer and Davis, commenting on the question of the bilateral 

 origin of the pineal organ, remark that : " The embryologist who con- 





ds — 



Fig. 151. — Transverse Section through the Diencephalon of Amia calva, 

 showing the stalk of the right plneal organ, and the left plneal 

 Organ in a Membranous Septum between Two Diverticula of the 

 Dorsal Sac (After Kingsbury.) 



The left pineal organ is connected by nerve-fibres with the habenular commissure ; 

 the left habenular ganglion is smaller than the right habenular ganglion. 

 ch. : chiasma. /. po. : left pineal organ. 



ds. : dorsal sac. r. hg. : right habenular ganglion. 



/. hg. : left habenular ganglion. st. : stalk of right pineal organ. 



templates the study of the pineal organ recalls the syllogistic statement : 

 ' The pineal organ is probably a sense-organ. The sense-organs are paired 

 structures ; ergo, the pineal organ should be a paired structure.' " They 

 also quote from an article by Dr. Ayers (J. Morph., 4, 1890, p. 228) : 

 " After a careful study of the Amphioxus eye-spot and related structures, I 

 have become convinced that the animal presents us with the earliest stage 

 in the phylogenetic development of the vertebrate eye." Although this 

 pigment presents a variety of forms the author finds " the most usual form 

 that of a slightly bilobed mass, the lobes being placed to the right and 

 left of the median line, so as to cover the roots of the first pair of cranial 

 nerves. . . . For greater functional power, the central median portion 

 of the pigment spot has grown upwards (dorsal) and carrying with 

 it a portion of the ventricular wall, has produced the median eye. . . . 

 The parietal pineal eye of the Cyclostomata and other vertebrates has been 



