FERTILIZATION IN PLANTS AND UNICELLULAR ORGANISMS 323 



or finally, ia unicellular organisms, are not of essential, but oiily of 

 secondary significance, however important they may be for securing 

 fertilization or conjugation in each special case. They are always 

 only adaptations to the special conditions, and only occur where they 

 are necessary to ensure the union, and always in such a manner that 

 the union of the two cells is facilitated. In most Infusorians such 

 a differentiation into male and female animals was not necessary, 

 because these organisms are very motile, and are thus readily able 

 to meet and unite; it was therefore sufficient for them to remain 

 hermaphrodite. The bell-animalcules, however, are sedentary, and 

 for them it was obviously an advantage that, at the time of conjuga- 

 tion, smaller, free-swimming, and also more simply organized in- 



FiG. 86. Conjugation of an Infusorian, Vmiiaila nebuUfira, showing sexual differen- 

 tiation of the whole organism. Aiter Greef. I, the ' miorogonidium ' or male 

 individual (mi) attaches itself to the ' raacrogonidium ' or female individual {ma) • 

 cv, contractile vacuole ; st, contractile stalk. II, the ciliated circle on the male individual 

 has disappeared. The male has become firinly embedded in the female by means of 

 a sucker-like retraction of its lower end. Ill, the fusion of the two individuals has 

 been completed ; the bristly residue of the male (ci) is about to be thrown off ; tlie 

 stalk (si) is contracted into a spiral. Magnified about 300 times. 



dividuals should arise, which were able to seek out the larger sedentary 

 forms. Here, then, as in many other unicellular animals, these little 

 male individuals only occur when they are necessary, that is, at the 

 time of conjugation. Similarly, in the green alga, Volvox, male and 

 female cells arise only at the time of conjugation, reproduction being 

 at other times effected by means of parthenogonldia, that is, by 

 elements which require no fertilization. 



As these differences are only adaptations to the necessity that 

 •the animals or cells shall find each other and unite, so also are all the 

 other differences of a sexual kind, the thousand-fold di0"erences 

 between the sperm-cell and the egg- cell, and the not less numerous 

 differences between male and female animals, both in ' primary ' and 

 especially in the diverse ' secondary ' sexual characters which we have 

 I. X 



