150 THE PROTOZOA 



3orl>ed and disappear, and the nuclei divide twice by karyokinesis 

 (Fig. 76, D), so that their number is quadrupled. The myxamceba 

 then undergoes multiple fission into as many cells as there are 

 nuclei in the plasmodium (Fig. 76, F), and each of these cells is a 

 gamete. The gametes now conjugate in pairs, and the zygotes 

 become encysted to form the spores. Within the spores the nuclei 

 of the gametes are stated to undergo a further process of reduction 

 before they fuse to form the synkaryon (Fig. 76, G). The syngamy 

 in Plasmodiopliora is stated to be a case of autogamy, but this 

 allegation assumes that the nuclei of the myxamcebse are sister- 

 nuclei derived all from the division of one original nucleus ; they 

 may equally well be nuclei of different origins brought together by 

 plastogamic fusions. 



The two examples selected, Arcella and PlasmodiopJiora, show 

 that the chromidia may represent generative chromatin in one 

 case, vegetative in another. Goldschmidt (57) has proposed to 

 distinguish these two conditions by a special terminology, retaining 

 the name " chromidia " (trophochromidia, Mesnil, 74) for those which 

 are purely vegetative, and coining a new term, sporetia (idio- 

 chromidia, Mesnil) for those of generative nature. It is more 

 convenient, however, to retain the term " chromidia " in its 

 original significance, to denote simply extraiiuclear particles of 

 chromatin, and to qualify the term by the adjectives " vegetative " 

 and " generative " when required (see also Goldschmidt, 41, p. 130)^ 

 The formation of vegetative chromidia, which are finally absorbed, 

 is a common phenomenon in many Protozoa ; it may take place 

 as a purely regulative process, as in Actinosphcerium during de- 

 pression-periods (p. 208), when hypertrophy of the nuclear apparatus 

 is corrected by the extrusion from the nuclei of chromidia, which 

 ultimately degenerate and become converted into masses of pig- 

 ment, and as such are eliminated from the protoplasm. 



The account given by Bott (103) of gamete-formation in the commoni 

 Pelomyxa (Amcebcea nuda, p. 227) describes a condition in which chromidia, 

 extruded from the nuclei, are partly vegetative, partly generative ; secondary 

 nuclei are formed from them, which later cast out a portion of their chromatin, 

 then give rise to the gamete-nuclei. After the secondary nuclei have been 

 purified in this way of their vegetative chromatin, the generative chromatia 

 remaining in each of them forms a karyokinetic spindle with eight chromo- 

 somes, and a reducing division follows by which each daughter-nucleus obtains 

 four chromosomes. The " pronuclei of the first order," resulting from t he- 

 first reducing division, divide again, forming a spindle with four chromosomes 

 which split, so that the " pronuclei of the second order " have also four chro- 

 mosomes. From the nuclei that have undergone reduction in this manner 

 the nuclei of the gametes arise in a somewhat remarkable fashion : the pro- 

 nuclei of the second order separate into two compact masses of chromatin ; 

 a vacuole is formed near them ; and the chromatin of the two masses wanders, 

 in the form of finely- divided granules, into the vacuolo to form the definitive- 

 pronucleus of the gamete, which forms a membrane when the process is- 

 complete. When formed the gametes wander out as Heliozoon-like ind ; 



