MANUAL OF ELEMENTARY ZOOLOGY 



Reproduc- 

 tion. 



separation. 



which leads an independent existence. It will be con- 

 venient to use the word "fission" to denote 

 the actual breaking away of the new body, 

 for reproduction is more than a mere act of 

 This will be seen if we consider it a little 



more closely. 



(a) As has been said, reproduction always involves the 

 fission of an existing body. Life never arises anew, but is 

 always passed on from one living being to another which 

 arises from it. A living being which divides to produce 

 others is a parent; those which it forms are offspring. 



(b) The offspring are always at first unlike the parent. 



There are, as we 

 shall see, certain 

 animals in which the 

 only evident differ- 

 ence between the off- 

 spring and the in- 

 dividual by whose 

 division they arose is 

 the necessary one of 

 size. But in the great 

 majority of cases there 

 is also an obvious 

 difference in form, 

 the offspring being 

 at first very unlike the 

 parent in structure. 

 This difference is 

 obscured in the case 

 of man and some 

 other animals, where 

 the offspring undergoes changes in the womb before birth 

 (Fig. i), but it is seen unmistakably in animals which are 

 born in the condition of an egg. In their immature con- 

 dition the offspring are known as reproductive bodies. 



(c) In spite of this unlikeness at starting, the offspring 

 become in time like the paretit from which they arose, owing 

 to a succession of changes which is sometimes straightfor- 

 ward, or direct, sometimes, as in the well-known case of 

 the butterfly, very roundabout, or indirect. Thus the life 



Fig. i. — The egg or "ovum'' from which 

 a human being is developed, highly 

 magnified. 



