364 THE BIOLOGY OF BIRDS 



(b) Embryological. — The egg of a crocodile, as we have 

 mentioned, is superficially like that of a goose and the internal 

 structure (with the white of egg and the yolk) is the same. 

 The early stages in development — the establishment of 

 three germinal layers, the nervous system, the notochord, 

 the primitive gut, the musculature, the body-cavity, and so 

 on, take place in the same way, and it is not till some days 

 have passed that the embryos of the reptile and the bird 

 becomes obviously different. The similarities in the early 

 stages of development prove genetic affiliation. 



There are many features in the development of birds 

 which point to a pedigree stretching into the obscure past 

 far beyond the ancestral reptiles. Thus on the side of the 

 neck of the embryo chick there are gill-clefts, opening out 

 from the pharynx. They have no respiratory significance ; 

 indeed they have no use at all except that the first becomes 

 the Eustachian tube from the ear passage to the back of the 

 mouth. But they persist in every embryo bird, tell-tale 

 evidences of a remote aquatic ancestry. 



A very interesting point in regard to the embryonic gill- 

 clefts has been demonstrated by Edward A. Boyden (1918). 

 Across the ventral surface of the neck in the embryo chick 

 and in some reptiles there is a band of tissue, which is 

 derived from the ventral union of the hyoid arches. From 

 its resemblance to the development of the gill-cover of 

 certain fishes and amphibians it may be justly called the 

 opercular or gill-cover fold. Now on the lateral margins of 

 this operculum, after it has grown backward to enclose at 

 least a potential gill-chamber, filamentous outgrowths are 

 observed on the under side. In reptiles they have a very 

 transitory existence, but in the chick they have a relatively 

 extensive and prolonged development. The filamentous 

 character of these outgrowths, their origin from gill-arches, 

 and their relation to the operculum point to the conclusion 

 that they are vestigial gill-filaments like those of fishes. The 

 persistence of gill-clefts in the embryo has been for a long 

 time familiar, but here we have a demonstration of vestigial 

 gill-filaments as high up the scale as birds. 



