DEVELOPMENT. 



451 



When the egg is laid, the segmentation is already completed 

 and the cicatricula has developed into the blastoderm. The 

 embryo, which later projects from the yolk, develops, as in 

 reptiles, the characteristic foetal membranes the amnion and 

 allantois. The duration of the embryonic development varies 

 according to the size of the egg and the relative development 

 of the young when hatched. The bird, when ready to come out. 

 breaks the blunt end of the shell by means of a sharp tooth 

 placed at the extremity of the upper beak. 



The young when hatched have essentially the organisation of 

 the adult animal, 

 although they may 

 be still far inferior 

 to it in the degree 

 of their bodily de- 

 velopment. While 

 the GalJi; Limicolae, 

 Lari, Ratitae,. etc., 

 have when hatched 

 a complete covering 

 of down, and are so 

 far advanced in de- 

 velopment, that 

 they at once follow 

 the mother on land 

 or into water and 

 there seek their own 

 food (praecoces) ; 

 others like the Pas- 

 seres, Columbinae, 

 etc. leave the egg membranes very early (altrices) ; they 

 are naked, or only covered with down in places, and incapable 

 of free locomotion or of feeding themselves, and remain for some 

 time in the nest, in which they are fed and tended by their 

 parents. 



The mental qualities of birds are incomparably higher than 

 those of reptiles. The higher development of the senses (sight) 

 renders them capable of a sharp discernment, with which is 

 combined a good memory. Under the guidance of its parents 

 the young bird in some instances learns to fly and sing ; it 



FIG. 247. Segmentation of the germinal disc of a fowl's 

 egg ; surface view (after Kolliker, from Claus). A germinal 

 disc with the first vertical furrow. E the same with two 

 vertical furrows crossing one another at right angles. C 

 and D more advanced stages with small central segments 



