ANIMAL BIOLOGY 



reptiles have a rudiment of a third eye which is located 

 as a rule near the middle of the roof of the skull. In 

 one species of reptile, Sphenodon, this eye is quite well 

 developed, but in all other forms it exists in a very de- 

 generate condition. This third eye was formerly con- 

 nected with a part of the brain known as the pineal 

 gland, a structure which is present, although rudimentary, 

 in nearly all vertebrates including man. Weidersheim 

 has recorded as many as 180 organs which are rudimentary 

 in the human body. 



ear 



, 



eye W V 



fll 



FIG. 236. Very young human embryos showing gill slits, gs and rudi- 

 ments of limbs, /. (After His.) 



It frequently happens that organs which have entirely 

 disappeared in the adult are represented by rudiments in 

 embryonic development. The upper incisor teeth are 

 absent in cattle, but rudimentary teeth are nevertheless 

 found in fcetal calves. In the whalebone whales teeth 

 are no longer present, but the embryos of these whales 

 have numerous teeth in a rudimentary condition which 

 later disappear. All of these rudimentary organs are 

 very naturally explained as structures which were useful 

 once, but which have become dwindled through disuse as 

 animals have adopted new habits of life. 



The evidence of the descent of higher from lower forms 

 furnished by embryology is often very striking. The 

 higher vertebrates without exception show the gill clefts 



