SEXUAL REPRODUCTION. ‘41 
Asexual reproduction is found most commonly in the 
lower phyla of animals, but ce//s 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 generations 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 (¢/ Parasitism, Chap. IX.). 
Methods of Asexual Reproduction :— 
A, FIssIon— Binary—two equal parts. 
(or division into equal parts). 
Multiple—many equal parts. 
&B. BuppInc— Internal, 
(or division into unequal parts). 
External. 
Sexual Reproduction.—It is characteristic of the 
multicellular animals or Méefazoa 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 :—(1) 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. Fhese processes are called respectively (1) 
Maturation and (2) Fertilisation. 
1. MaruraTion.—The essential reproductive organs 
are called gonads and give rise to cells known as the 
primitive germ-cells. The male element is produced in an 
organ called the ¢es#is and the female element in an ovary. 
In the case of the male, the male element or spevmatozoon 
is produced by rapid increase to double its size of the 
