FERTILIZATION IN PLANTS AND UNICELLULAR ORGANISMS 323 



or finally, in unicellular organisms, are not of essential, but only 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 thej^ 

 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- 



Fio. 86. Conjugation of an Infusoiian. Vortlcella nehuUfera, showing sexual diffeion- 

 liation of tlie whole organism. Alter Greef. J, the ' n:iicrogonidium ' or male 

 individual (mi) attaches itself to Ihe ' macrosronidium ' or female individual {ma); 

 (T, contractile vacuole ; sif, contractile stalk. //. the ciliated circle on the male individual 

 has disappeared. The male has become firmly 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 {ct) is about to be thrown off; the 

 stalk {st) 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 parthenogonidia, 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 difierences of a sexual kind, the thousand-fold differences 

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

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

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

 I. X 



