1 68 FOOT-NOTES TO EVOLUTION. 



into bundles of minute antherozooids (sixty-four to a 

 hundred and twenty-eight). The remaining cells of the 

 colony, remain in a vegetative condition, and eventu- 

 ally die. In reproduction, one of the antherozooids 

 fuses with one of the oospheres, a resting zygote is 

 formed from which develops later a new colony. Thus 

 in the Volvox colony we meet with a differentiation into 

 somatic or vegetative cells and reproductive cells, a dif- 

 ferentiation which persists through all the multicellular 

 plants and animals. 



A much larger series of forms might be cited to 

 illustrate the phenomena of multiplication among uni- 

 cellular organisms, which would show all stages of gra- 

 dation in the relative size of the conjugating cells from 

 those in which both are of equal size and are equally 

 active, to such forms as Volvox, in which a great dif- 

 ference in size exists, the larger, the oosphere, being 

 non-motile and laden with food material, the smaller, 

 the antherozooid, having the cytoplasm reduced to 

 a very small amount and being endowed with high 

 mobility. 



In multicellular organisms we meet with a continua- 

 tion of the same facts. The animal egg is a single cell 

 laden with a large amount of food yolk, 

 Reproduction in ^^^ ^^^^ ^^ nucleus and cytoplasm 



as the living elements. For the develop- 

 ment of this egg, conjugation with another germ cell, 

 derived from a different individual, is necessary. This 

 germ cell is the spermatozooid, a minute cell consisting 

 of nucleus and centrosome with a small amount of cyto- 

 plasm modified primarily into an organ of locomotion, 

 the tail. A physiological division of labour is here met 

 with which admirably meets two diametrically opposed 

 requirements. The one of these demands that the con- 

 jugating cells be highly motile, and consequently small, 



