268 THE BIOLOGY OF BIRDS 



remarkable vitality in the embryo, and this is well illustrated 

 by the ready way in which wounds of a chick's blastoderm 

 are healed. The process can be watched for hours in 

 hanging-drop preparations. In wounds of the extra- 

 embryonic part of the blastoderm all the three germinal 

 layers take part in the process, but the ectoderm and the 

 endoderm are somewhat more active than the mesoderm. 

 There is de-differentiation or simplifying ; then fusion of 

 cells as if they were helping one another ; then re-differentia- 

 tion or re-establishment of the old complexity. Wounds in 

 the embryo itself are mainly healed by the ectoderm cells. 

 There is de-differentiation of the ectoderm and amoeboid 

 migration of the ectoderm cells over the wound. The 

 mending is just like a special case of the regenerative capacity 

 which developing structures possess in a high degree 

 (Poynter, 19 19). 



The inheritance of an organism consists of a large number 

 of initiatives or " factors " which develop when the appro- 

 priate liberating stimuli are present. They express them- 

 selves as the gramophone record does when the spring is 

 released. The initiatives or factors are germinal differentia- 

 tions representative of the organism's characters, and some 

 of them have what one might call a strong inertia. If the 

 minimal vital conditions are present, the potentialities unfold. 

 Thus up to a certain point the complex structure of the bird's 

 lung is established although the lung is not in use. Beyond 

 a certain point, however, the lung will not develop unless 

 the young bird breathes actively in the open air. And this 

 is important, for the fullness of development depends in 

 most cases on some measure of activity. It is not a passive 

 unfolding, it is an active trading with time and circumstance. 

 A fully developed character is always a product of the in- 

 herited " nature " and the appropriate " nurture," which 

 includes use and exercise, as well as food and suitable 

 surroundings. 



Another biological idea that must be kept in mind in 

 thinking of development is the general tendency to re- 

 capitulate stages in racial evolution. In the differentiation 



