THE LIFE-HISTORY OF BIRDS 267 



paired structures one behind the other, they came to take up a 

 fan-shaped arrangement ; a process of evolution which may be 

 compared to the development of the " composite " flower-head 

 from a primitive "spike". 



There is no hard-and-fast line, no set order, in the chain of 

 of these events. Some appear and disappear before the com- 

 pletion of embryonic life ; some begin at the close of this period 

 and terminate soon after hatching, while others do not appear 

 till near the end of the nestling period, or at the beginning or 

 end of the adolescent period, while, finally, others are what we 

 call " adult characters ". These last, it may have been re- 

 marked, seem to be subject to the same laws of growth — 

 adolescence, maturity and senility — which govern the body as 

 a whole : the senile character being represented by vestiges, 

 embryonic or adult. 



It is obvious then that we cannot regard the embryonic 

 period as a phase of development sharply divided from the 

 post-embryonic, and this is nowhere more evident than among 

 the birds. By way of an illustration let us take the case of the 

 Megapodes. These birds while yet within the shell develop 

 and shed the nestling down plumage characteristic of the Game- 

 birds to which the Megapodes belong, so that the young emerge 

 from the egg fully fledged, and with the wing quills so com- 

 pletely developed that they are enabled to fly therewith within 

 an hour of leaving the shell. 



Of the characters which are peculiar to embryonic life, char- 

 acters developed solely to serve the ends of development, we 

 need mention here but one or two — the egg-tooth and the em- ■ 

 bryonic membranes. The former is a small, conical, calcareous 

 body generally loosely attached to the tip of the upper jaw and 

 used by the young bird to break away the shell wall in order 

 to escape therefrom. Soon after hatching this drops off". 



The embryonic membranes are the allantois and the amnion. 

 The former is a specially developed sac arising from the gut 

 which, pushing its way outwards, ultimately curves over the 

 embryo to underlie the shell, serving at one and the same time 

 as a receptacle for the urine and a respiratory organ, the latter 

 function being performed by reason of the fact that it is richly 

 supplied with blood-vessels which absorb oxygen through the 

 porous shell. Towards the end of the incubation, when the em- 



