THE PINEAL EYES OF CYCLOSTOMES 



193 



authors and others describe only a single diverticulum which spreads 

 forward, backward, and also laterally. In the later stages of development 

 the stalk becomes elongated and its lumen disappears, the two terminal 



Fig. 136. — Schematic Diagram designed to show the Relations of the Two 

 Epiphyses in Vertebrates. (After J. B. Johnstone.) 



ca. : anterior commissure. np. 1 : nerve of anterior epiphysis. 



cp. : posterior commissure. np. 2 : nerve of posterior epiphysis. 



D.s. : dorsal sac. Par. : paraphysis. 



Ep. 1 : anterior epiphysis. t. : tectum. 



Ep.- : posterior epiphysis. vel. : velum. 



vesicles being carried forward away from the original site of origin of the 

 diverticulum, namely, between the anterior and posterior commissures. 

 On the left side a part of the left habenular ganglion is carried forward 

 with the anterior or ventral terminal vesicle, and forms the " cushion " 

 described by Ahlborn on which the vesicle rests (Fig. 22, Chap. 3, p. 28) ; 

 the remaining part of the ganglion is left in its original position, the 

 two parts of the ganglion being connected by a strand of nerve-fibres 

 called the " pineal or habenular tract." The whole of the larger right 

 habenular ganglion appears to remain in its original position. It is 

 connected with the superficial vesicle or " parietal organ " by the " pineal 

 nerve " and by some commissural or chiasma fibres with the posterior 

 part of the left habenular ganglion ; these fibres constitute the habenular 

 or anterior commissure. The right pineal nerve (Dendy, Studnicka) is 

 connected with the posterior commissure and right bundle of Meynert as 

 well as with the right habenular ganglion. 



The intermediate stages of development between the adult condition 

 and the embryonic condition have, so far, not been completely filled in. 

 Studnicka, however, showed that the pineal nerve of a 35-mm. Ammocoetes 

 (Fig. 137, a) extended backward from the parietal organ to the posterior- 

 commissure, and that the nerve-fibres at this stage were enclosed through- 

 out the whole length of the nerve in a nucleated sheath which was continu- 

 ous anteriorly with the wall of the parietal organ . In transverse sections 

 of ;he pineal nerve, obtained from an Ammocoetes of similar age, the 

 proximal end of the nerve (Fig. 137, d) consisted almost exclusively of 

 13 



