270 THE CELL 



stationary cannot be foretold from their structure, for it depends 

 solely and entirely upon their position and their consequent role 

 during the process of fertilisation. Thus the portions which are 

 situated nearest to the zone of thickening become the migratory 

 nuclei (III, 1m, 5m); the two conjugating bodies exchange 

 these migratory nuclei; these pass each other across the proto- 

 plasmic bridge, which has been formed for this purpose. During 

 this exchange, the male migratory nuclei possess the structure 

 of spindles (IF, 5m, 1m). After the exchange has been com- 

 pleted, each male nucleus coalesces with a stationary or female 

 nucleus, which is also in the form of a spindle (IF, 1% 5w), so 

 that now each animal possesses only one spindle the division 

 spindle (vt) if we disregard the segments of the chief nucleus, 

 and the paranucleus, which are gradually undergoing disinte- 

 gration. 



The similarity to the process of fertilisation, as it occurs in 

 Phanerogamia and animals, is striking. In Paramaecia, the 

 stationary and migratory nuclei unite to form a division spindle, 

 just as in plants and animals the egg- and sperm-nuclei unite to 

 form the germinal nucleus. The division spindle serves to replace 

 the old nuclear apparatus, which is becoming dissolved. It in- 

 creases considerably in size (Fig. 148 F, /). The chromatin ele- 

 ments inside it arrange themselves into a plate ; they then divide 

 and move apart towards opposite ends of the spindle, almost up to 

 the poles, thus forming the daughter-plates (F, right t' t"). The 

 two halves remain united for a considerable time by a connecting 

 strand. They then develop in a roundabout fashion into chief 

 nucleus and paranucleus ; in Paramsecium aurelia (Fig. 148 FI) 

 for example, the daughter-spindles (t', t"), which have been 

 formed out of the primary division spindles, re-divide, and so pro- 

 duce four spindles (FI), two of which develop into paranuclei 

 (nk 1 , nk'), whilst the other two coalesce to form the chief nucleus 

 (pt). Thus, in Infusoria, " fertilisation brings about a complete 

 re-organisation of the nuclear apparatus, and at the same time of 

 the Infusorian" (Richard Hertwig). 



Sooner or later, after the exchange of migratory nuclei, the two 

 individuals separate from one another (Fig. 148 FI, FII). 

 A longer period is necessary for the reabsorption of the useless 

 portions of the nucleus, and for their replacement by new for- 

 mations. The individuals, which have thus become rejuvenated, 

 have regained the capacity of multiplying enormously by means 



