NATURAL HISTORY NOTES 53 



the bird from beast and reptile, wing, foot, 

 crop and gizzard now first commencing to 

 take form and shape, skull becoming bird- 

 like and nose showing signs of developing 

 into a beak. By the end of the first week 

 matters are so far advanced that scientists 

 can already detect the differences that 

 distinguish the various orders or natural 

 groups into which birds are divided. 



On the ninth day feathers begin to 

 protrude from the skin, covering most of 

 the body four days later, although they 

 are contained in thin horny sheaths until 

 a day or two before hatching. During 

 this, the second week of incubation, 

 the generic characteristics or special dis- 

 tinguishing features of the family to 

 which the bird belongs may be recog- 

 nized. 



The determination of sex is a fascinat- 

 ing problem, and although the laws which 

 govern its operation have not as yet been 

 established with any finality, still the re- 

 corded results of many recent experiments 

 indicate a solution which fits in nicely 



