94 



THE PROTOZOA 



According to the achromatinic theory, the centrosome is " an 

 individualized portion of the achromatinic nuclear substance " 

 (Hertwig, 66), a kinetic centre on which the movements of the 

 framework are focussed. The essential and primary constituent 

 of the centrosome is the centriole, and so long as the centrosome 

 remains intranuclear, as in perhaps the majority of Protozoa, it 

 consists of the centriole alone. When, however, the centrosome 

 becomes extranuclear, as in many Protozoa and almost universally 



H 



Fio. 49. Paramoeba etihardi : stages of the life-cycle. A, Amoeba in the vegetative 

 stage: N., nucleus; n.k., " Nebenkern " ; d., ingested diatom. B, G, D, 

 Stages in the multiplication of the encysted amoeba ; in B the Nebenkern 

 has divided up, the nucleus is still undivided ; in the nucleus has divided 

 up into a number of daughter-nuclei, each of which has paired with a daughter- 

 Nebenkern ; in D the body has divided into a number of daughter-cells, each 

 containing a nucleus and a Nebenkern. E, A free-swimming flagellula, derived 

 from one of the daughter-cells in D, and containing a nucleus and a Nebenkern. 

 F, G, H, I, Four stages of the division of a flagellula ; in F the Nebenkern is 

 dividing ; in the two halves of the Nebenkern have placed themselves on 

 each side of the nucleus, which is preparing for division ; H, stage of the 

 nuclear spindle with the two halves of the Nebenkern at each pole ; in / the 

 nuclear division is nearly complete, and the body is beginning to divide. After 

 Schaudinn (81), all figures magnified about 500 diameters. 



in the cells of the higher animals and plants, accessory cytoplasmic 

 elements may be added to the centriole to form a centrosomic 

 complex. A point still undecided, on the theory that centrosomes 

 are of achromatinic origin, is whether or no these bodies can be 

 formed, in some cases, in the cytoplasm also, as maintained by 

 some authorities. On Hertwig's view, mentioned above, that the 

 achromatinic substance of the nucleus is identical in nature with the 

 ground-substance of the general protoplasm, it follows that material 

 for the formation of the centrosome must be present in the cyto- 



