The Nature of Animal Life. i 3 



the nature of reproduction in the simple unicellular amoeba. 

 The reproduction of the constituent cells in the complex 

 multicellular organism, during its natural growth or to 

 make good the inevitable loss consequent on the wear and 

 tear of life, is of the same character. 



When we come to the higher organisms, reproduction 

 is effected by the separation of special cells called egg-cells, 

 or ova, from a special organ called the ovary ; and these, 

 in a great number of cases, will not develop into a new 

 organism unless they be fertilized by the union with them 

 in each case of another cell — the sperm-cell — produced by a 

 different individual. The separate parents are called male 



h 



Fig. 5. — Egg-cell and sperm-cell. 

 a, ovum or egg; b, spermatozoon or sperm. 



and female, and reproduction of this kind is said to be 

 sexual. 



The wonderful thing about this process is the power of 

 the fertilized ovum, produced by the union of two minute 

 cells from different parents, to develop into the likeness of 

 these parents. This likeness, however, though it extends 

 to minute particulars, is not absolute. The offspring is not 

 exactly like either parent, nor does it present a precise 

 mean between the characters of the two parents. There 

 is always some amount of individual variability, the effects 

 of which, as we shall hereafter see, are of wide importance. 

 We are wont to say that these phenomena, the transmis- 

 sion of parental characteristics, together with a margin of 



