EVOLUTION AND DISEASE. 



sharks and the Alpine salamander, or by means of the 

 yolk sac forming adhesions to the oviducal wall, as in 

 Miistelus Icevis, or even by means of the tail (Ccecilia 

 compressicauda). 



It is not unjustifiable to hold the allantois responsible 

 for the abolition of gills in sauropsida and mammalia. 

 This is much more probable than to attribute the change 



to the evolution of lungs from 

 a swim-bladder, for functional 

 gills and lungs co-exist in such 

 forms as the mud-fish (Lepido- 

 siren) and ceratodus. 



The history of the pineal 

 eye is an instructive instance. 

 Connected with the vertebrate 

 mid-brain is a structure known 

 as the pineal body, which 

 has long puzzled anatomists. 

 Many investigators have re- 

 garded it as vestigial ; that is, 

 it was of some functional value 

 in the ancestors of exist- 

 FIG. 20. The head of a Lizard ing vertebrata. The truth of 



this opinion has been demon- 

 strated by the admirable re- 

 searches of De Graaf and Baldwin Spencer. 



On the dorsal aspect of the skull in lizards a 

 small opening exists, known as the parietal foramen. 

 In some lizards as, e.g., Varanus the situation of 

 this foramen is indicated by a bright scale (fig. 20). 

 On making a longitudinal section of the head, so as 



