CHANGES IK THE EGG DTJR1XG INCUBATION. 335 



to it from the yolk ; when the incubation has advanced con- 

 siderably, the albumen loses almost the whole of its water and 

 salts ; these seem to be transferred to the yolk, which admits 

 of explanation, for the vitellary sac bursts and draws the 

 albumen, now changed into a thick mass, into it. ]3y this 

 accession of matter, the yolk enlarges during the first half of 

 the period of incubation, but becomes thinner ; the incessant 

 demand upon it, however, for materials for the growth of the 

 embryo, causes it again to shrink and to become more consis- 

 tent towards the end of the period ( 494). The proportion 

 of chemical elements of the vitellus and white vary consider- 

 ably ; the quantity of phosphorus contained in the albumen 

 lessens, but increases in the yolk, and again appears in com- 

 bination with oxygen and calcium as a phosphate of lime, 

 which in the period of ossification is plentifully required 

 for the consolidation of the bones ; as the quantity of lime 

 contained in an egg at the time it is laid is extremely small, 

 and becomes very large at a subsequent period, the earth must 

 be acquired in some way with which we are not at present 

 well acquainted. As it is not very probable that the lime is 

 derived from the shell, it may perhaps be produced from other 

 matters under the influence of the organic agencies ; the same 

 may be said of the iron, the quantity of which increases 

 greatly during incubation.] * 



* The whole of this article on the development of the chick is from 

 Professor Wagner, Elements of Physiology, p. 84, et seg. It forms a 

 valuable complement to the chapter on Embryology. ED. 



