I.

Isogamy

1.

The union of two gametes

II.

Syngamy

2.

Two gametes, one larger than the other

III. 

Heterogamy  

3.  

Two kinds of gametes, one motile and other non motile

IV.

Anisogamy

4.

Two kinds of morphologically distinct gametes

V.

Oogamy

5.

Both gametes morphologically indistinguishable

I  

II

III

IV

V

1.  

1     

2     

3     

4     

5   

2.

4

1

2

3

4 

3.

5

1

2

4

3

4.   

3

1

2

4

5