THE LOB OS A 



the second phase, in which the radiating spicules do not occur, 

 reproduction -may also occur in a manner similar to that of the first 

 phase, but at the conclusion of vegetative growth the pseudopodia 

 are withdrawn, all foreign bodies and excreta are expelled, and a 

 cyst is formed. The nuclei then divide rapidly by repeated mitoses 

 to form an immense number of minute nuclei. These nuclei become 

 the nuclei of minute biflagellate swarm-spores (gametes), which escape 

 from the gelatinous investment of the cyst, and after conjugation 

 give rise to small individuals of the first phase. 



In Pelomj/xa, a multinucleate freshwater rhizopod (Fig. 14), repro- 

 duction is effected by simple or multiple fission during the vegetative 



period of life, but at certain times, 

 after a complicated series of 

 nuclear divisions in which a re- 

 duction in the number of chro- 

 mosomes occurs, uninucleated, 

 heliozoan-like swarm-spores escape 

 which conjugate to form a zygote, 

 and this encysts. From the cyst 

 a uninucleated amoebula escapes, 

 which by growth and multipli- 

 cation of the nucleus gradually assumes the typical Pelomyxa form. 



In the preparation of the nuclei for the formation of the gametic 

 nuclei, a considerable part of the chromatin is discharged into the 

 cytoplasm, and from that which remains eight chromosomes are 

 formed on the equatorial band of a central spindle (Fig. 11, a). Two 

 successive divisions take place, the first of which is regarded as a 

 reduction division, and the 

 second as an equation division. 

 The chromatin of the four 

 chromosomes of this last 

 division collect together in 

 two lumps, and a transparent 

 globular vacuole appears in 

 their immediate neighbour- 

 hood. This vacuole gradually 

 fills with minute granules 



Fio. 10. 



Zygote of Pelomyia palustris. a, encysted. 

 I, after escape from the cyst. (After Bott.) 



chr. 



FIG. 11. 



Nuclear formation in Pdomyxa, a, the spindle 

 of the reduction division with eight chromo- 

 somes, b, the nucleus (A T ) of the gamete forming 

 in a clear vacuole. ch, the chromatin lumps of 

 which rapidly increase in Size the last nuclear division. (After Bott.) 



and gives rise to the nucleus 



of the gamete (Fig. 11, b). The chromatin lumps at the same time 



dwindle and eventually disintegrate. 



In Centropyxis, one of the Thecamoebida, binary fission occurs 

 by the protrusion and division of the protoplasm preceded by 

 amitotic division of the nucleus. One portion of the divided proto- 

 plasm with one nucleus returns to the old shell, the other forms a 

 new shell but of a larger size. It does not seem certain Avhether 



