226 PHYSIOGRAPHY. [chap. 



young plant within the pea. It is in a quiescent state, and 

 its activity must be roused by an external influence. This, in 

 the case of the egg, is simply a certain amount of heat 

 (which is ordinarily furnished by the warmth of the body of 

 the parent), the supply of nourishment being yielded by the 

 matter stored up within the egg itself, in the yolk and 

 white. Under these circumstances, the cicatricula enlarges 

 by the growth and multiplication of its cells, and rapidly 

 extends over the surface of the yolk. Part of it rises up 

 and becomes fashioned into a rude resemblance of the body 

 of a vertebrated animal, in which the head, trunk, and tail 

 gradually become more and more recognizable ; while the 

 limbs grow out like buds, at first without much likeness to 

 either legs or wings. 



As the yolk is used up in the construction of the embryo, 

 it diminishes as the latter increases ; the young bird be- 

 comes larger and larger, acquires its feathers, and puts on 

 more and more completely the characters of a pigeon. At 

 last, it leaves the shell, and grows to the full size of its 

 kind. In the adult state the female bird possesses an organ 

 termed the ovary, in which nucleated cells, the primitive ova, 

 which correspond with the embryo cells of the plant, are 

 developed. Each of these grows and becomes invested by 

 the materials of the egg ; and, before it is laid, it undergoes 

 a process of division, whereby it is converted into the em- 

 bryonic patch, or cicatricula, from which this series of 

 changing forms has proceeded. 



Thus the pigeon is developed from a simple nucleated 

 cell, by a process of evolution, similar in principle, how- 

 ever dissimilar in its results, to that which gives rise to the 

 pea-plant. The adult pigeon is an aggregate of modified 

 cells, descended by repeated division from the primitive 

 egg cell ; and this aggregate assumes a series of successive 



