FACTORS TX ONTOGENY 



261 



more or less complicated variations of the phenomena just outlined 

 for ('l/ilo</on. In all of them the aim of the conjugation is the same, 

 the exchange of a certain amount of nuclear substance between the 

 two conjugating individuals, and the same physiological effect is 

 reached, a rejuvenescence, as it were, of the two organisms which 

 manifests itself in renewed vigor of growth and multiplication. 



" In some of the lowest forms of unicellular life for example, the 

 Sehizomycetes or bacteria and their allies- this necessity for con- 

 jugation does not 

 appear to exist, but 

 for the vast ma- 

 jority of forms this 

 cyclical law of de- 

 velopment holds 

 good. In the Pro- 

 tozoa no division 

 into somatic and 

 germinal cells is 

 found, both func- 

 tions being united 

 in the one cell which 

 forms the whole 

 body of the or- 

 ganism. In the Met- 

 azoa, however, this 

 differentiation has 

 taken place ; the germinal cells are set apart for the preservation of the 

 race; the somatic cells carry on their various functions for a time, 

 grow old, die, and disappear, certain of the germ cells alone surviving 

 in the production of new individuals. On the borderland between 

 the unicellular and the multicellular organisms, however, stand cer- 

 tain colonial forms, which show an exquisitely graded series of steps, 

 from the conditions of unicellular multiplication to those of the multi- 

 cellular forms." (McFarland.) 



In the many-celled animals the egg is a single cell laden 

 with a large amount of food yolk, and made up of nucleus and 

 cytoplasm as the living elements. For the normal development 

 of this egg, conjugation with another germ cell, derived from a 

 different individual, is usually necessary. This germ cell is the 

 spermatozooid, a minute cell consisting of nucleus and centro- 

 18 



FIG. 147. Gonium pectorale, a simple colonial Protozoan, 

 composed of sixteen cells holding together in a single 

 layer or plate: A, The whole colony; B, a single cell; e, 

 eye spot; cl, chloroplast; n, nucleus; v, vacuole. (After 

 Campbell.) 



