THE NUCLEUS 87 



for the cell ; then, when cell-division takes place, the kinetonucleus 

 first divides, and the two products of its division place themselves 

 on each side of the trophonucleus and act as its centrosomes, as 

 described by Franca and Athias (56)* ; (6) the trophonucleus may 

 have a centrosome of its own, lodged in the karyosome, in addition 

 to the centrosome-blepharoplast in connection with the kineto- 

 nucleus ; this is probably the most usual condition with two sub- 

 ordinate variations, according as the centrosome-blepharoplast is 

 lodged within the kinetonucleus, as in Leishmania tropica (Wenyon, 

 84), or is situated close beside it, as in most trypanosomes ; in either 

 case the kinetonucleus.and trophonucleus divide quite independently 

 of one another, as commonly seen ; (c) it is possible, but perhaps 

 not very probable, that hi some cases there may be a blepharoplast 

 for the flagellum distinct from the centrosomes of the two nuclei ; 

 such a condition, perhaps, occurs in Trypanoplasma. In all cases 

 alike, division is initiated by the centrosome from which the 

 flagellum arises; next the kinetonucleus, and lastly the tropho- 

 nucleus, divide. 



The various forms of flagollar insertion described in the foregoing para- 

 graphs admit of a simple and uniform phylogenetic explanation. Starting 

 with a non-flagellated organism in which a simple protokaryon contains a 

 single centriole (Fig. 44, O 3 ), we may suppose the flagellum at its first origin 

 to grow out from the centriole in the nucleus (0 b ). No such condition is 

 actually known amongst flagellates, though it may be compared to the origin 

 of the axopodia from a central grain in an Actinophrys-type of Heliozoon 

 (see below) ; in the flagellates the centrosome-blepharoplast always, ap- 

 parently, moves out of the nucleus, either remaining in close proximity to it 

 (l a ) or becoming quite independent of it (l b ), the two variations of the first 

 type. 



The second, type may be derived by division of the centrosome-blepharo- 

 plast to form the definitive centriole and the blepharoplast ; the latter may 

 also remain in close proximity to the nucleus (2 a ) or become quite independent 

 of it (2 b ). 



The third type may be supposed to arise from the hypothetical primitive 

 condition (0) by supposing that, not the blepharoplast-centrosome alone, 

 but the whole nucleus, divides to form two nuclei of unequal size and distinct 

 function, the trophonucleus and kinetonucleus, each with its own centriole 

 (3 a , 3 b ). The centriole of the kinetonucleus, which is at the same time the 

 blepharoplast, may either remain within the kinetonucleus (3 b ) or come out 

 of it (3 C ) ; its relations to the kinetonucleus are parallel to those of the centro- 

 some-blepharoplast to the nucleus in types l a and l b . Or, on the other hand, 

 the centrosome-blepharoplast may divide into a definitive kinetonuclear 

 centrosome and a true blepharoplast (3*). The condition with only a single 

 centriole for both the nuclei may, if it exists, be derived from 3 a or 3 b by 

 supposing that the trophonuclear centrosome becomes atrophied. 



When a blepharoplast exists independently of the nuclear 

 apparatus, it may retain the form of a single grain or basal granule 

 of the flagellum, when this organ is single, or it may multiply to 



* The statements of Franca and Athias are not, however, confirmed by Lebedeff 

 (468), and it may be doubted whether any species of trypanosome or other " binu- 

 cleate " exists which has but a single division-centre in the cell. 



