42 EVOLUTION AND DISEASE. 
sharks and the Alpine salamander, or by means of the 
yolk sac forming adhesions to the oviducal wall, as in 
Mustelus levis, or even by means of the tail (Cecilia 
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 (Lepzdo- 
siven) and ceratodus. 
The history of the pineal 
Sou eet . eye is an instructive instance, 
R ip iaudnce xy, Connected with the vertebrate 
ST ing 
ORG 
mid-brain is a structure known 
as the pineal body, which 
has long puzzled anatomists. 
Many investigators have re- 
carded it as vestigial ; that is, 
it was of some functional value 
in the ancestors of exist- 
Fic. 20.—The head of a Lizard ing vertebrata. The truth of 
(rete ee, ate ME 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, ¢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 
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