92 THE PROTOZOA 



the axial filaments of the pseudopodia are centred, while the nucleus 

 is displaced to one side and becomes excentric in position ; when 

 the cell enters upon division, the central grain becomes the centro- 

 some(Fig. 64). 



From the condition seen in Acanthocystia, it is not difficult to explain the 

 state of things which has been described by Zuelzer (86) in the remarkable 

 form Wagnerella (p. 246). Here also the buds formed possess each a single 

 nucleus containing a centriole ; in this condition they may multiply by fission 

 with mitosis, in which the centriole functions as a centrosome. When the 

 buds develop into the adult form, a centriole is extruded from the nucleus 

 to form the central grain. The organism attaches itself, and the body becomes 

 divided into three regions head, stalk, and basal plate (Fig. 48). The 

 nucleus travels down into the basal plate, while the central grain remains 

 in the head and functions as the kinetic centre of the pseudopodia, becoming 

 very complicated in structure. It consists of a centrosome surrounded by a 

 sphere, which is perhaps of the nature of archoplasm, but is stated to be 

 rich in plastin ; when the pseudopodia are extended the sphere shows well- 

 marked radial striations. From the centrosome minute granules are budded 

 off. which pass along the striations of the sphere to its surface, and from these 

 granules arise the delicate axial filaments of the pseudopodia ; the basal 

 granules are therefore comparable to the ring of blepharoplasts in Lophomonas. 

 When the pseudopodia are retracted, the basal granules lie within the sphere, 

 immediately surrounding the centrosome, and the radial striations of the 

 sphere vanish. The centrosome itself varies in structure at different times, 

 going through cyclical changes, but usually shows a distinct central granule or 

 centriole. 



When Wagnerella divides by fission, the central grain and the nucleus 

 divide independently, and the central grain does not act as a centrosome 

 for the dividing nucleus, which contains its own centriole. In this form, 

 therefore, the central grain, though centrosomic in origin and nature, loses 

 its primitive relation to the division of the nucleus, a^nd becomes specialized 

 exclusively as a kinetic centre for the organs of locomotion, a course of evolu- 

 tion perfectly parallel to that which has been traced above for the blepharo- 

 plasts in their relation to flagella. 



While there can be but little doubt as to the centrosomic nature 

 of the blepharoplasts or basal granules of the flagella, and of the 

 central grains on which the pseudopodia of the Heliozoa are centred, 

 the true nature of the basal grains of cilia, on the other hand, is 

 less certain. The majority of those who have studied them in 

 Ciliata are of opinion that they have nothing to do with centro- 

 somes (compare Maier, 73, and Schuberg, 44, and see p. 443, infra) ; 

 but there are certain observations which indicate that the basal 

 granules of the cilia have a connection with (Collin, 50), or an 

 origin from (Entz, 53), the nuclear apparatus, in which case they 

 may be of the same nature as the multiple blepharoplasts of such 

 a form as Lophomonas. Hertwig (66) considers that the basal 

 grains of the cilia may be of centrosomic nature, and that, if they 

 have no connection with the nucleus, they afford support for the 

 view that centrosomes can arise from the cytoplasm as well as 

 from the nuclear framework. In view of the great structural 

 similarity between cilia and flagella in other respects, it seems 



