DISTRIBUTION QF REPRODUCTIVE METHODS 263 



reinvigorate the process. Conjugation occurs chiefly among 

 the unicellular organisms. It is found also among some multi- 

 cellular plants, but in no multicellular animals. 



B. Fertilization, or sex union proper. When the uniting 

 cells are unlike, one being much larger than the other, their 

 union constitutes fertilization, or true sex union. The larger 

 of the two uniting bodies is the egg and the smaller the sperm. 



ORIGIN OF SEX UNION 



Conjugation and sex union are evidently closely allied. In- 

 deed, some organisms show a type of reproduction that is 

 halfway between conjugation and true sex union, and give 

 us an idea as to what was probably the origin of sex. We 

 have already studied Pandorina (Fig. 28), in which we found 

 an animal multiplying by the union of two similar cells; 

 but the two cells, although similar, are not exactly alike. Both 

 are rounded cells, both provided with flagella which enable 

 them to swim; but one is a little larger than the other, and 

 when union occurs it is always that of a larger with a smaller 

 cell. Whether this is a true sex union or a conjugation it is 

 difficult to decide. 



A step further in the line of sex differentiation is found in 

 Eudorina. This organism is much like Pandorina, and is 

 composed of a cluster of rounded flagellate cells, inclosed in 

 jelly; Fig. 123 A. They multiply by a method of simple divi- 

 sion as does Pandorina (shown at A), and in addition they 

 multiply by cell union. In the latter case the cells break up 

 into many small parts, after which there is a union of cells. 

 But here the uniting cells are very unlike. Some of the cells, 

 shown at C, D, E, break up into a large number of small flagel- 

 late cells, of an elongated shape. The other cells of the colony 

 do not divide, but slightly enlarge and remain spherical. Even- 

 tually one of the small flagellate cells comes ^n contact with 

 one of the rounded ones and the two unite. Here there is a 

 plain suggestion of egg and sperm, and consequently of a true 



