THE PINEAL BODY 211 



c. Relative constancy of the epiphyseal complex with reference 

 to other structures of the pineal region. The phyletic constancy 

 of the epiphyseal complex, when considered in conjunction with 

 the other derivatives of the diencephalic roof-plate, brings to 

 light a fact of no little significance. It has already been shown 

 how constant the epiphyseal complex, either in its entirety or 

 in some of its parts, is in the vertebrate phylum, and this be- 

 comes further emphasized by the fact that this structure alone 

 of all the elements derived from the roof-plate presents such 

 undeniable constancy. If compared with one of the most con- 

 spicuous roof-plate derivatives, the paraphysis, the epiphyseal 

 complex stands out in marked contrast. The paraphysis is 

 present in its highest state of evolution in the middle of the 

 vertebrate series; that is to say, in amphibians and in older 

 reptiles. It is a conspicuous organ, showing but little differ- 

 entiation in cyclostomes and in fishes generally. In ophidians, 

 birds, and mammals it is absent. The inference which may be 

 drawn from these facts seems to be that the roof-plate of the 

 interbrain is capable of developing a structure which, when it no 

 longer subserves any purpose, ceases to exist. When the phy- 

 letic constancy of the epiphyseal complex is compared with that 

 of the paraphysis, it would seem evident that this very constancy 

 argues a demand on the part of the organ for the presence in the 

 animal of this complex or some of its parts. 



To a less degree, the comparison in favor of the pineal organs 

 may be drawn with reference to the velum transversum, telen- 

 cephalic chorioid plexus, and dorsal arch. None of these show 

 such a marked tenacity as the epiphyseal complex, a fact which 

 but serves to emphasize the significance of the relative constancy 

 among the structures derived from the diencephalic roof-plate. 



d. Relative constancy of the several parts of the epiphyseal com- 

 plex, with the predominance of the proximal portion. Since each 

 organ of the epiphyseal complex presents three more or less 

 well-defined portions, namely, the proximal portion, the stalk, 

 and the end-vesicle, it would be interesting to note the relative 

 constancy of the several parts in the phyletic series in order to 

 ascertain, if possible, which of these is the most fundamental 



