THE VITAL PROPERTIES OF THE CELL 



271 



of division, until again the necessity for a new " conjugation 

 epidemic " arises. 



The conjugation period at the same time causes a somewhat 

 lengthy cessation of multiplication in the life of the Infusorian, 

 as Maupas, for instance, has plainly shown in the case of Onychro- 

 dromus grandis, where, if the temperature is kept at from 17 to 

 18, an interval of six and a half days occurs between the com- 

 mencement of conjugation and the first subsequent division. 

 During this period, if conjugation is not taking place, a single 

 individual, when provided with sufficient nourishment, divides 

 thirteen times ; that is to say, it produces from 7,000 to 8,000 

 descendants. 



In most Infusoria, as in the cases described here, both con- 

 jugating individuals behave in the same way, each functioning 

 towards the other as male and female, that is to say, both impart- 

 ing and receiving. Fixed forms of Infusoria, however, such as 

 Vorticellce, etc., behave in an interesting and somewhat different 

 fashion. 



The Epistylis umbellavia 

 (Fig. 149) may serve as an 

 example. When a conjuga- 

 tion period is approaching, 

 several individuals of the 

 colony of Vorticellae divide 

 rapidly and repeatedly, thus 

 producing a generation of 

 individuals (r) very inferior 

 in size to the mother organ- 

 ism. Other individuals of 

 the colony remain undivided 

 and of normal size. The 

 former are called microga- 

 metes, and the latter macro- 

 gametes ; they differ from 

 one another sexually. 



Each microgamete detaches itself from its stalk, swims round 

 in the water, and after a short time attaches itself to a macro- 

 gamete in order to conjugate with it (Fig. 149 k). Changes 

 occur in the nuclear apparatus similar to those described in detail 

 above in the Param&scium, and migratory nuclei are exchanged here 

 also. However, the macrogamete alone continues to develop, the 



Fig. 149. Epistylis umbellaria fafter Graeff; 

 from R. Hertwig, Fig. 142) : portion of a colony 

 in the act of conjugation ; r microzoids pro- 

 duced by division ; k microgametes in conjuga- 

 tion with macrogamete8. 



