EMBRYOLOGY 233 



are others not so easily followed. The external gills, hidden under a 

 fold of skin called the operculum, disappear early, to be replaced by in- 

 ternal gills which are developed on the endodermal lining of the gill 

 slits. The internal gills are lost later, and their function served by 

 lungs, which have all the while been developing. The jaws are provided 

 with a horny armature, serving as teeth, but these are shed and the mouth 

 increases greatly in size. The intestine, from the early tadpole stage a 

 very long and much coiled tube, is greatly shortened. And there are 

 other changes. 



Such metamorphoses do not occur in all animals. In many groups, 

 all of the organs that the adult possesses are present in recogniza- 

 ble form at birth or hatching. While these organs may change their 

 relative sizes during post-embryonic life, the alterations are so slight that 

 no one would mistake the group to which the new-born animal belongs, 

 even if he were ignorant of its life history. Young birds and mammals 

 (the hairy animals) fall within this category. In such animals there is, 

 of course, no metamorphosis. 



Homology.^It can hardly fail to attract attention that in the account 

 of organogeny given above, the narrative is a generalized one. It 

 is couched in terms that make it applicable, not to a single form, but 

 on the whole to vertebrates in general. The nervous system of one verte- 

 brate begins much as does the nervous system of any other vertebrate. 

 The eye of a bird develops as does the eye of a frog. The early ear 

 also is about the same, whether found in a reptile or mammal. 



This similarity of the first appearance of embryonic structures occurs 

 even when the adult organs are strikingly different. The forelimb of man 

 and the wing of a bird are very different from one another in the adult 

 condition, especially in the hands; but in the embryo, the earliest limb 

 buds are almost identical. An even greater difference exists between the 

 adult fore and hind limbs of a bird. When compared, bone for bone, 

 there is scarcely a point at which there is not a distinct difference. Yet 

 the wing and leg could be interchanged in the early embryo, and few 

 persons would detect the substitution. Nearly every other organ which 

 is common to the various groups of vertebrates arises in the same funda- 

 mental manner in its beginnings. 



Structures arising in the same manner in the embryo are held to be 

 homologous. The term homology may be defined as similarity of origin 

 in evolution, and is best determined from similarity of origin in the 

 embryo. The description of organogeny in this chapter could be made 

 general because homology is prevalent. Indeed, in animals of the same 

 kind, as in the group of vertebrates, similarity of embryonic origin of 

 organs is the rule. Differences are usually slight. Within a smaller 

 group of animals, as in the birds, the similarity is even closer, and the 

 differences less significant. 



