460 COMPARATIVE ZOOLOGY 



the fish, with a system of blood vessels similar to those 

 in the fish's gills. These openings afterward close and 

 disappear, and the most of the blood vessels waste away. 

 Thus, a condition which is permanent in the fish is only 

 transitory in the higher vertebrates, and it can be ex- 

 plained only on the supposition that during their devel- 

 opment these forms repeat the phases through which 

 their ancestors passed in the course of their evolution. 

 Embryology thus corroborates paleontology in showing 

 that the earliest vertebrates were fishlike. All of the 

 vertebrates possess at a certain period of their embryonic 

 life a notochord or rod of cartilage, which is later re- 

 placed by a vertebral column in all forms except Amphi- 

 oxus, in which the organ persists. This indicates that 

 vertebrates are descended from an amphioxus-like ances- 

 tor. Until their embryological history was learned, 

 Ascidians were considered to be mollusks. The dis- 

 covery of a rudimentary notochord at an early stage of 

 their development showed that they are closely allied to 

 the vertebrates. The adult halibut, turbot, sole, and 

 other "flat fish " have both eyes on the same side of the 

 head. In the embryonic stage the eyes are placed as 

 in other fishes, showing that in their ancestors the eyes 

 were in the usual position. A West Indian frog, Hylo- 

 des, which lays eggs on land, passes through its tadpole 

 stage in the egg. It has a large tail, like the ordinary 

 tadpole, and gill slits in place of functional gills. The 

 tail wastes away almost entirely, and lungs are devel- 

 oped before hatching occurs, the young animal thus 

 entering upon its terrestrial mode of life unhampered by 

 organs suited only to an aquatic existence. Its develop- 

 ment shows that it is descended from ancestral forms 

 which had both tail and gills. 



The tendency of animals to. pass through stages of 

 development in which they temporarily exhibit features 



