SEXUAL REPRODUCTION. 41 



Asexual reproduction is found most commonly in the 

 lower phyla of animals, but cells are produced asexually 

 throughout the whole kingdom. 



In many instances, one or more asexual generations 

 may alternate with the sexual method. This phenomenon 

 is known as Alternation of generatiojis or Metagenesis, It is 

 usually found in organisms whose life-history is very varied, 

 and involves such dangers at certain periods that a multi- 

 plication immediately prior thereto is necessary to the 

 continuance of the species {cf. Parasitism, Chap, IX.). 



Methods of Asexual Reproduction : — 



A. Fission — Binary — two equal parts. 



(or division into equal parts). 



Multiple — many equal parts. 



B. Budding — Internal. 



(or division into unequal parts). 



External. 



Sexual Reproduction. — It is characteristic of the 

 multicellular animals or Metazoa that they reproduce 

 sexually. In sexual reproduction a portion of the parent 

 is liberated, as in asexual reproduction, and gives rise to a 

 fresh organism. The main differences are these : — (i) The 

 liberated portion is never more than a single cell (which is 

 called the sexual element) and is produced in special organs. 

 (2) This single cell completely fuses with another single 

 cell, liberated in the same fashion from another individual, 

 but differing in shape and structure. The fused cell so 

 produced divides into a multicellular individual by repeated 

 cell-division. These processes are called respectively (i) 

 Maturation and (2) Fertilisation. 



I. Maturation. — The essential reproductive organs 

 are called gonads and give rise to cells known as the 

 pr lenitive germ- cells. The male element is produced in an 

 organ called the testis and the female element in an ovary. 

 In the case of the male, the male element or spermatozoon 

 is produced by rapid increase to double its size of the 



