224 BULLETIN OF THE 



the term be limited as I have suggested that it ought to be. The dis- 

 tinctness of the eitiphysial vesicle from the proximal portion of the epi- 

 physis in the adult Phrynosoma is without exception^ so far as my 

 observations have gone ; and if it is regarded as having been derived 

 from the epiphysis, then we have two vesicles instead of one that have 

 arisen in this way, and the difficulty of explaining the nature and 

 function of the whole structure is correspondingly increased. 



Jn his recent paper, Leydig ('90) has expressed the belief that there 

 are two forms of parietal organs. He says : " From the posterior por- 

 tion of the embryonic thalamencephalon (Zwischenhirn), especially in 

 Lacerta agilis, two thick-walled vesicles (Blasen) bnd out just in the 

 middle line, lying one behind the other and springing from a common 

 root (eincm Wurzelpunkte). The anterior vesicle gives rise to the 

 parietal organ, and the posterior one constitutes the epiphysis (Zirbel)." 

 It is only, he says, from the anterior of these two vesicles (Dlasen) that 

 a vesicle (Blase) becomes cut off, and attains an eye-like character; the 

 posterior one ends in the expanded blind terminal portion of the epi- 

 physial thread (Zirbelfaden). But Selcnka ('90) informs us, in a still 

 more recent communication, that, after studying the development of the 

 brain in a large number of reptiles and other vertebrates, he is unable 

 to confirm Leydig's statement as to the origin of the parietal eye. He 

 does find, however, in all cases, an evagination from the dorsal wall of the 

 fore brain very similar to the one that forms the epiphysis from the roof 

 of the thalamencephalon ; also that the two structures elongate pari 

 passu, the epiphysis becoming directed upward and forward, while the 

 anterior evagination, which he calls the " paraphysis," becomes directed 

 upward and backward. After the parietal vesicle is cut off from the 

 epiphysis, the distal end of the paraphysis grows in between the vesicle 

 and the end of the epiphysis from which it was detached, and the vesicle 

 comes to lie on the paraphysis as on a pillow. 



The relation of the two structures in the adult he does not know. 

 C. K. Hoffmann ('85) has also described an evagination from the roof 

 of the brain at the place of transition from the fore brain to the thala- 

 mus, which he calls the ependyma, — the beginning of the choroid 

 plexus, — and he says that in the grown animal "it comes to take a 

 not inconsiderable part in the formation of the epiphysis." Although 

 there is nothing in the brief papers of either Leydig or Selenka to indi- 

 cate whether or not the additional more anterior evagination seen by 

 them is the same as that described by Hoffmann, yet, since all have 

 studied the same forms, viz. of the genus Lacerta, it seems quite prob- 



