268 A HISTORY OF BIRDS 



bryo is able to breathe by means of its lungs, the allantois shrivels 

 up and is ultimately cast off with the shell : but its narrowed 

 stalk, from the gut to the navel, remains for some time as the 

 " urachus " upon the inside of the body wall. 



The amnion is a delicate membrane enveloping the em- 

 bryo, so that it lies within a closed sac which is filled with 

 fluid, the liquor amnii, whose purpose is to preserve the embryo 

 from injury and pressure, and to permit the free use of the 

 growing limbs. 



We cannot, it is contended, draw any hard-and-fast line 

 between embryonic and post-embryonic development, since, as 

 we have already seen, phases of development which in some 

 birds are post-embryonic are in others embryonic. Con- 

 sequently, the phases usually considered as belonging to 

 embryology are here dealt with as phases in the common 

 life-history (ontogeny) which begins with the fertilisation of the 



egg. 



Concerning embryological characters little is said here, in- 

 asmuch as these demand intimate knowledge of abstruse facts 

 that only indirectly concern the theme of this book. But in 

 the course of development birds pass through many stages 

 reminiscent of earlier ancestors, and carrying us back even be- 

 yond the reptiles, the stock from which the birds are evidently 

 derived. Thus the presence of gill arches in the embryo must be 

 regarded as survivals of an amphibian stage common to both 

 reptiles and birds. And similarly in the development of 

 organs such as the heart, brain and kidneys — to go no further — 

 we have like evidence of pre-reptilian stages. 



We may proceed then to review the essential phases of the 

 life-history which are strictly post-embiyonic. 



The first of these phases is that presented by the nestling, 

 which as we have seen may leave the egg in a relatively ad- 

 vanced stage of development, or in a condition of utter help- 

 lessness; while between the two extremes there are many 

 gradations. 



The time required to attain maturity varies enormously. 

 Thus some species, as in the Penguins, Albatrosses and Secre- 

 tary-birds, may remain "nestlings" for many months — the 

 larger Penguins, as the King Penguin, for example, remaining 

 in the downy stage for as long as twelve months ; and the 



