260 OUTLINES OF EVOLUTIONARY BIOLOGY 



skin in the parietal foramen in the middle of the roof of the 

 skull. It may to some extent still be functional, though probably 

 of .far less importance than in the extinct amphibia and reptiles 

 of the Palaeozoic and Mesozoic periods, in which the presence of 

 a comparatively large parietal foramen indicates that the eye was 

 better developed. In the case of the tuatara it has been demon- 

 strated by anatomical and embryological investigation that the 

 remaining pineal eye is the left-hand member of the original 

 pair. In the lampreys it appears to be the right-hand member. 



Turning now to the frog, we sometimes find in this animal a 

 minute light-coloured spot on top of the head (Fig. 115), between 

 the paired eyes. This marks the position of a little sac-shaped 



vesicle which lies 

 beneath the skin and 

 is known as Stieda's 

 organ, consisting merely 

 of a number of undiffer- 

 entiated cells surround- 

 ing a central cavity and 

 attached to the end of a 

 kind of stalk (Fig. 116). 

 The vesicle represents 

 the functionless vestige 

 of a pineal eye and the 

 stalk probably repre- 

 sents its disappearing 

 nerve. 



In the birds and mammals all trace of eye-like structure has 

 disappeared from this region, but a vestige of one or both of the 

 pineal eyes is probably to be recognized in the so-called pineal 

 gland lying on the roof of the brain, which attained such celebrity 

 in the seventeenth century owing to its identification by the 

 philosopher Descartes as the seat of the soul. 



In studying the mammalian de_niition we again meet with 

 plenty of illustrations of vestigial structures. The whale-bone 

 whales (Balaenidae) have no teeth in the adult at all, but in the 

 foatus vestiges of teeth can be found imbedded in the gums. The 

 young Australian duck-billed Platypus (Ornithorhynchus) his 

 vestigial teeth which are entirely replaced by horny pads in the 

 adult. The third molar in the lower jaw of the dog (Fig. 108, 

 AI, m. a ) ' is, as we have already seen, evidently in a vestigial 



FIG. 115. Head of Frog (Ran<t temporaria}, 

 showing Stieda's Organ between the lateral 

 Eyes. (From Studnicka, after Stieda.) 



