INTRODUCTION. 9 



and from which the young emerge in different degrees of de- 

 velopment, those of the Gallinaceous and Duck tribes being 

 the most matured, and capable of running about and picking 

 food in the course of a few hours. The forms of eggs vary : 

 in most instances the egg is not exactly oval, but has one 

 of the ends smaller and more pointed than the other, which 

 allows a larger number of them to be conveniently arranged 

 in the nest for incubation. Where birds lay fewer eggs, it 

 has been remarked these more nearly approach a strictly oval 

 form ; but in the instance of the Guillemot, which lays but 

 one egg, and that often greatly exposed upon the jutting 

 ledges of the seaside cliffs, we may observe that the great 

 length of the egg, and tapering pointed form of the smaller 

 end, seem designed to preserve it from readily rolling off the 

 narrow platform on which it lies undefended by any nest 

 whatever. If this egg is set in motion on a table, it will be 

 perceived, instead of rolling far, its tendency is to revolve 

 around its apex as a centre. 



We shall not delay to trace minutely the changes which 

 take place within the egg during the process of incubation. 



The embryo, with some appearances of the principal blood- 

 vessels, may be discovered by the aid of a microscope within 

 forty hours after incubation ; on the fourth, fifth, and sixth 



