PARAPHYSIS AND PINEAL REGION IN REPTILIA 363 



is homologous with fibers crossing the third ventricle in the supra 

 commissure in amphibia. 



In Lacerta the commissure is well marked and appears first in 

 an embryo of 17 mm., fig. 10. It becomes a well marked tract 

 in higher stages, figs. 11-13. In Chrysemys it appears first in an 

 embryo of 9.5 mm., fig. 20. As I mentioned above it is a curious 

 fact that in these two forms and also in Necturus the commissure 

 appears at the same stage in which the first traces of the lateral 

 choroid plexus are seen. Cameron also describes a well marked 

 commissure in reptiles. See also Burckhardt, Francotte, Dendy, 

 Beraneck and others. 



In birds the commissure was first described by Dexter ('02) in 

 the chick, figs. 2, 3 and 9. It appeared in a chick of 19.5 mm. 

 Cameron finds it first in chicks of ten days incubation close in 

 front of the epiphysis. Cameron (15) calls attention to the 

 fact that in Cunningham's Text-book of Anatomy, p. 506, men- 

 tion is made of fibers from the stria medullaris that pass across 

 the median line in front of the stalk of the epiphysis to reach the 

 ganghon habenularum. Cameron considers these fibers homolo- 

 gous with the superior commissure of lower forms. He found in 

 studying sections of this region stained by the Pal-Weigert method 

 that this commissure was well defined and divided into an anterior 

 and posterior part by a cleft. He noticed also that some of these 

 fibers passed into the epiphysis at its base on the opposite side, 

 figs. 8 and 9. This show^s that there is a distinct decussation 

 of fibers here as was seen in amphibia. He observed furthermore 

 that the fibers belonging to the striae medullaris lay in the ante- 

 rior portion, while the posterior part was occupied by fibers arising 

 in one ganglion habenula and passing to the other or to the 

 epiphysis. This hinder part alone he thinks to be homologous 

 with the commissure of lower forms. 



The commissure has been described in pigs by Minot (72) , who 

 states that it is very constant in all classes of vertebrates, and 

 Neumayer (75) has shown it in the sheep and rabbit (101), fig. 47. 

 It alw^ays appears after the posterior commissure is well developed 

 except in Ammocoetes, where it appears before the posterior, 

 Kupffer (57). 



