PROTOZOA 



as compared with what previously obtained. Now the 

 two groups diverge, and in many cases a striated appear- 

 ance of the achromatin substance between the two groups 

 of loops of chromatin is observable (H). In some cases 

 (especially egg-cells) this striated arrangement of the 

 achromatin substance precedes the separation of the loops 

 (G). The striated achromatin is then termed a " nucleus- 

 spindle," and the group of chromatin loops (Fig. I., G, ) 



FIG. I. Karyokinesis of a typical tissue-cell (epithelium of Salamander) after 

 Flemming and Klein. The series from A to 1 represent the successive stages 

 in the movement of the chromatin fibres during division, excepting G, which 

 represents the " nucleus-spindle " of an egg-cell. A , resting nucleus; D, wreath- 

 form; E, single star, the loops of the wreath being broken; F, separation of the 

 star into two groups of U-shaped fibres; H, diaster or double star; I, comple- 

 tion of the cell-division and formation of two resting nuclei. In G the 

 chromatin fibres are marked a, and correspond to the phase shown in F ; they 

 are in this case called the ' ' equatorial plate " ; 6, achromatin fibres forming the 

 nucleus-spindle; c, granules of the cell-protoplasm forming a "polar star. 1 ' 

 Such a polar star is seen at each end of the nucleus-spindle, and is not to be 

 confused with the diaster H. 



is known as "the equatorial plate." At each end of 

 the nucleus-spindle in these cases there is often seen a 

 star consisting of granules belonging to the general proto- 

 plasm of the cell (G, c). These are known as " polar stars." 

 After the separation of the two sets of loops (H) the 

 protoplasm of the general substance of the cell becomes 

 constricted, and division occurs, so as to include a group of 

 chromatin loops in each of the two fission products. Each 

 of these then rearranges itself together with the associated 

 achromatin into a nucleus such as was present in the 

 mother-cell to commence with. This phenomenon is termed 

 " karyokinesis," and has been observed, as stated above, 

 in a large variety of cells constituting tissues in the higher 

 animals and plants. 



There is a tendency among histologists to assume that 

 this process is carried out in all its details in the division 

 of all cells in the higher plants and animals, and accordingly 

 to assume that the structural differentiation of achromatin 

 plasma and chromatin nucleus-fibres exists in the normal 

 nucleus of every such cell. If this be true, it is necessary 

 to note very distinctly that the nucleus of the Protozoon 

 cell-individual by no means conforms universally to this 

 model. As will be seen in the sequel, we find cases in 

 which a close approach is made by the nucleus of Protozoa 

 to this structure and to this definite series of movements 

 during division (Fig. VIII. 3 to 12, and Fig. XXV.); and 

 a knowledge of these phenomena has thrown light upon 

 some appearances (conjugation of the Ciliata) which were 

 previously misinterpreted. But there are Protozoa with a 

 deeply-placed nucleus-like structure which does not pre- 

 sent the typical structure above described nor the typical 

 changes during division, but in which on the contrary the 

 nucleus is a very simple homogeneous corpuscle or vesicle 

 of more readily stainable protoplasm. 



The difficulties of observation in this matter are great, 

 and it is proportionately rash to generalize ; but it appears 

 that we are justified at the present moment in asserting 

 that not all the cells even of higher plants and animals 



exhibit in full detail the structure and movement of the 

 typical cell-nucleus above figured and described; and accord- 

 ingly the fact that such structure and movement cannot 

 always be detected in the Protozoon cell-nucleus must not 

 be regarded as either an isolated phenomenon peculiar to 

 such Protozoon cells, nor must it be concluded that we have 

 only to improve our means of analysis and observation in 

 order to detect this particular structure in all nuclei. It 

 seems quite possible and even probable that nuclei may 

 vary in these details and yet be true nuclei. Some nuclei 

 which are observed in Protozoon cell-bodies may be regarded 

 as being at a lower stage of differentiation and specializa- 

 tion than are those of the epithelial and embryonic cells 

 of higher animals which exhibit typical karyokinesis. 

 Others on the contrary, such as the nuclei of some 

 Eadiolaria (vide infra), are probably to be regarded as 

 more highly developed than any tissue cell-nuclei, and will 

 be found by further study to present special phenomena 

 peculiar to themselves. In some of the highest Protozoa 

 (the Ciliata) it has lately been shown that the nucleus 

 may have no existence as such, but is actually dispersed 

 throughout the protoplasm in the form of fine particles of 

 chromatin-substance which stain on treatment with car- 

 mine but are in life invisible (84). This diffuse condition 

 of the nuclear matter has no parallel, at present known, in 

 tissue-cells, and curiously enough occurs in certain genera 

 of Ciliata whilst in others closely allied to them a solid 

 single nucleus is found. The new results of histological 

 research have necessitated a careful study of the nucleus 

 in its various stages of growth and division in the cell- 

 bodies of Protozoa and a comparison of the features there 

 observed with those established as " typical " in tissue-cells. 

 Accordingly we have placed the figure and explanation of 

 the typical cell-nucleus in the first place in this article for 

 subsequent reference and comparison. 



CORTICAL SUBSTANCE. The superficial protoplasm of 

 an embryonic cell of an Enterozoon in the course of its 

 development into a muscular cell undergoes a change 

 which is paralleled in many Protozoa. The cortical layer 

 becomes dense and highly refringent as compared with the 

 more liquid and granular medullary substance. Probably 

 this is essentially a change in the degree of hydration of 

 the protoplasm itself, although it may be accompanied by 

 the deposition of metamorphic products of the protoplasm 

 which are not chemically to be regarded as protoplasm. 

 The differentiation of this cortical substance (which is not 

 a frequent or striking phenomenon in tissue-cells) may be 

 regarded as an ectoplastic (i.e., peripheral) modification 

 of the protoplasm, comparable to the entoplastic (central) 

 modification which produces a nucleus. 



The formation of " cortical substance " in the Protozoa 

 furnishes the basis for the most important division into 

 lower and higher forms, in this assemblage of simplest 

 animals. A large number (the Gymnomyxa) form no 

 cortical substance ; their protoplasm is practically (except- 

 ing the nucleus) of the same character throughout. A 

 nearly equally large number (the Corticata) develop a 

 complete cortical layer of denser protoplasm, which is 

 distinct from the deeper medullary protoplasm. This 

 layer is permanent, and gives to the body a definite shape 

 and entails physiological consequences of great moment. 

 The cortical protoplasm may exhibit further specialization of 

 structure in connexion with contractile functions (muscular). 



ECTOPLASTIC PRODUCTS CHEMICALLY DISTINCT FROM 

 PROTOPLASM. The protoplasm of all cells may throw down 

 as a molecular precipitate distinct from itself chemical 

 compounds, such as chitin and horny matter and other 

 nitrogenized bodies, or again non-nitrogenous compounds, 

 such as cellulose. Very usually these substances are 

 deposited not external to but in the superficial proto- 



