THE PINEAL BODY 91 



development. One or the other element of the epiphyseal com- 

 plex may show the tendency toward the development of visual 

 characteristics, but in no other form do both of these elements 

 take on these features so suggestive of visual function. 



Differences observed in the epiphyseal complex of the various 

 species of cyclostomes already investigated. Although all of the three 

 European forms of Petromyzon have been carefully studied by 

 several investigators, the differences between them are not 

 striking. This statement also applies to the North American 

 form, Lampetra wilderi, described by Johnston 195 in 1902. 



1. Petromyzon planeri. Ahlborn ('83), 2 Beard ('89), 18 Whit- 

 well, ('88) 421 and Studnicka ('93). 384 The epiphyseal complex as 

 a whole is not separated as far from the brain as in other forms, 

 due to the fact that the paraphysis and dorsal sac are but little 

 developed. The parietal fossa is very shallow and is absent in 

 Ammoccetes as is also the white pigment. 



2. Petromyzon fluviatilis. Ostroumoff ('87), 291 Owsiannikow, 

 ('88), 295 Leydig, ('96) 239 and Studnicka ('99). 388 In this form the 

 evagination of the roof is very high and the fossa in the skull of 

 considerable depth. The atrium contains a definite syncytium 

 made up of processes not only from the retinal cells, but also 

 from those situated in the pellucida as well. 



3. Petromyzon marinus. Studnicka ('99). 388 Although the 

 dorsal sac is extremely high, the depression in the skull is no 

 deeper than in the case of Petromyzon fluviatilis. 



4. Petromyzon wilderi. Johnston ('02). 195 In this form the 

 stalk of the pineal organ has not the significance as in other 

 forms, for the pineal nerve is absent and the stalk contains no 

 nerve fibers. 



5. Mordacia mordax. Spencer ('90). 369 In this form the 

 pineal organ presents a thin, pigmented upper wall correspond- 

 ing to the pellucida of Petromyzon, and a thicker ventral wall in 

 the form of a retina. No definite statement is made as to the 

 presence of an atrium, although the lumen of the organ is said 

 to be filled by a coagulum. There is no evidence of any para- 

 pineal organ, but on the surface of the head, midway between 

 the paired eyes, there is a parietal spot. 



