EXPLAINING THE CONTRADICTIONS. 133 



these two methods of direct and indirect construc- 

 tion. Instead of repeating the whole past history, it 

 attempts in some features to build the chick in the 

 most direct way, and drops many of the earlier 

 stages. But it has not succeeded in freeing itself 

 from them all, the tendency to reproduce its earlier 

 history being strong enough to preserve many of 

 the more important stages. A notochord is devel- 

 oped which represents the primitive backbone of 

 the vertebrates. It appears in the young chick, but 

 is subsequently completely lost, nothing remaining 

 in the adult chick to indicate its existence. The 

 young chick develops gill-slits which are of no. use to 

 the embryo or the adult. They consequently soon 

 close up and disappear, with one exception, which 

 becomes converted into an entirely different struc- 

 ture, the Eustachian tube connecting the mouth 

 with the inner ear. Blood-vessels to supply these 

 slits also appear, but having no use now that respi- 

 ration is no longer carried on here, soon change 

 their position and come to supply other parts of 

 the body. In these respects the embryology of the 

 chick can be compared to the building of a steam- 

 ship, where the builder does not indeed make first a 

 Roman trireme complete, but goes so far in that 

 direction as to make in his boat the long tiers of 

 openings for oars. Finding, however, after he gets 

 his steam-engine into the boat, that oar-holes are no 

 longer of any use, he either closes them up or con- 

 verts them into port-holes. Oars he never makes, 

 since he discovers that they will not be needed, and 

 to make them would be sheer loss of time. 



