EMBRYOLOGY AND REPRODUCTrON. 79 



that some of the Reptiles are possessed of a third 

 eye, median in position. . The discovery of a fabled 

 Cyclops, a veritable giant with an eye in the middle of 

 his forehead, would hardly have been more startling. 

 Yet the rudimentary^ structure to which this interpre- 

 tation has now beyond, doubt been assigned, called the 

 'pineal gland, has been known to anatomists for cen- 

 turies: its use has always been doubtful, and we owe to 

 Descartes the suggestion that in the human being it 

 was the seat of the soul ! In certain reptiles it clearly 

 shows a structure corresponding to that of a rudimen- 

 tary eye. 



The description of the longitudinal structure in the 

 early chick, called the notochord, led us to consider 

 the links between vertebrates and invertebrates : the 

 egg itself, as a structure, serves as a reminder that 

 there are links, equally interesting, between the highest 

 group of the vertebrata, the mammalia, and the other 

 classes. The absence of eggs, on account of the pro- 

 duction of the young by birth instead of by hatch- 

 ing, has always been noted as the great characteristic 

 of the mammalia, although there are exceptional 

 instances of viviparous species in many groups of 

 animals. But in 1884, the lowest members of the 

 mammalian group, the Ornifhodelphia, were dis- 



'■ When an imperfect structure appears to be the inherited 

 trace of an ancestral structure which was complete and 

 perfect, it is often spoken of as vestigial (from Lat. vestigium, 

 footprint or trace) rather than rudimentary. The latter word 

 is retained above, as more familiar to the general reader. 



