228 FREDERICK TILNEY AND LUTHER F. WARREN 



The supposition advanced by Hertwig 175 and others that the 

 pineal process in birds and mammals undergoes metamorphoses 

 which give rise to an organ of a glandular or follicular structure 

 has little to support it. Peytoureau 308B maintained that in 

 the evolution through the vertebrate phylum the pineal body 

 has become partly atrophic and partly metamorphosed in such 

 a way as to cause a modification in the connection with the 

 nerve centers. Ultimately, it has taken on the characters of 

 an epithelial organ, in fact, a highly vascular gland represented 

 in the higher mammals by the pineal gland and its peduncle. 

 To assume that an actual process of metamorphosis, in a literal 

 sense, from a visual organ to a glandular structure, is respon- 

 sible for the differences between the parietal eye and the pineal 

 gland seems wholly unsatisfactory. If, however, this view has 

 reference to a deflection in the ontogenetic process, as a result of 

 which the pineal anlage in certain forms, instead of giving rise to 

 a visual structure, produces a gland, there may be some justifi- 

 cation of ascribing these changes to metamorphosis. Yet, even 

 in this sense, to attribute the differences between the parietal 

 eye of Sphenodon and the pineal gland of the bird to such an 

 indefinite process of alteration does little more than apply a 

 term to the process without offering an explanation for it. 



Certain investigators, among them Rabl-Ruckhard, 322 Ahl- 

 born, 2 and Spencer, 368 regard the pineal body as an unpaired 

 parietal eye which, in many classes, for example, reptiles, appears 

 to be tolerably well preserved, but in most vertebrates is in a 

 process of degeneration. This theory goes a step further than 

 that which regards the pineal body as a vestige. According to 

 the former view, the pineal differences between such forms as 

 possess a parietal eye and those in which no such structure 

 develops are attributed to a process of degeneration, while the 

 latter theory ascribes them to an arrested development. Evi- 

 dence of degeneration in the higher vertebrates is difficult to 

 discern. The figures already cited in reference to the human 

 pineal gland (p. 158) makes it hard to believe that a retrograde 

 process is present, even in the late periods of life. The appear- 

 ance of brain sand in itself is not sufficient to justify such a con- 



