THE PINEAL EYES OF CYCLOSTOMES 195 



matral sheath of the stalk which is continuous with that of the parietal 

 organ, and covers a limiting neuroglial membrane is also considered by 

 Studnicka as evidence of the pineal nerve being formed from and repre- 

 senting the stalk of the primary diverticulum. The pineal nerve of 

 Petromyzon thus appears to differ from that of Lacerta (see p. 243), in 

 which the nerve-fibres are developed separately and independently of the 

 original stalk (Figs. 172, 173, Chap. 20). The explanation of this 

 difference probably lies in the circumstance that while in Cyclostomes the 

 stalk remains for a sufficiently long period to form a ready made pathway 

 for the nerve-fibres to take, in their growth from the retina to the habenular 

 ganglion ; in amphibia and in reptiles the parietal organ is cut off from 

 the stalk before the growth of the nerve-fibres commences. 



On the left side the statement that true nerve-fibres pass from the 

 left terminal vesicle to the anterior part of the left habenular ganglion 

 is said by Studnicka not to have been confirmed, but whether the fibres 

 joining the two parts are true nerve-fibres or not, the connection between 

 the vesicle and the ganglion obviously represents the stalk of the left 

 pineal organ, in which, if the vesicle should retain any sensibility to 

 light, the nerve-fibres would be developed. 



With regard to the term " pineal tract " which is rightly applied to 

 the strand of nerve-fibres connecting the two parts of the left habenular 

 ganglion (Fig. 137, B, and Fig. 22, Chap. 3) ; if the ganglion cells of the 

 right parietal organ of Petromyzon are regarded in the same light as those 

 in the retina of a lateral vertebrate eye, namely as retinal, then the right 

 pineal nerve of the lamprey, like the " optic nerve " of the human eye, is 

 strictly speaking a " nerve tract," similar to inter-ganglionic nerve- 

 tracts in the central nervous system generally, and the distinction implied 

 between the terms " pineal nerve " and " pineal tract " is misleading. 

 From the morphological standpoint the ganglion-cells around the atrium 

 and elsewhere in the retina of the right parietal organ of Petromyzon 

 probably correspond to those of the anterior part of the left habenular 

 ganglion and the pineal nerve of the right pineal organ represents and 

 corresponds to the pineal tract of the left organ. On the right side the 

 anterior ganglion cells appear to have been incorporated in the retina ; on 

 the left side they have remained as a separate ganglion outside the 

 vesicle, but connected with it by a wide stalk of neuroglial tissue, which is 

 surrounded by a constricting fold of pia mater. 



The Development of the Parietal Organ of Petromyzon 



According to Studnicka's description of the development of the two 

 parietal organs, the " pineal organ " is the first to develop (Fig. 138). It 

 appears at the posterior part of the roof of the interbrain in the form of a 



