1893- NUCLEUS IN UNICELLULAR ORGANISMS. 175 



As typically represented, they are oval or suboval, are possessed 

 of one or more flagella, and have an average long diameter of about 

 the I -4000th of an inch. 



Two fair representatives of the group as re-examined during the 

 last four years with the advantage of our immensely-improved object- 

 glasses and eye-pieces are represented in Fig. i. A represents a form 

 about the i -4000th of an inch in long diameter, and Ba kindred form 

 the I -5000th of an inch measured in the same way. They are in the 

 drawing magnified 1,800 diams. 



A is distinctly nucleated, and trails a flagellum when swimming, 

 but (as in the drawing) often " anchors " it to the " floor" of the stage ; 

 and from this position acts by springs upon the masses of decomposing 

 matter around it. The form B simply swims either forward or back- 

 ward by means of its flagellum at either end. 



The life-cycle of A may be taken as representative, and will show 

 what was actually known by us before the investigations upon the 

 nucleus made within the last four years with our present apochromatic 

 microscopic illumination and apochromatic objectives. 



Taking A, Fig. i, as the normal form, in certain conditions of the 

 organism now definitely known to us, a sudden opening of the "beak " 

 to which the flagellum is attached {a. Fig. 2, C) would arise partly 

 dividing the flagellum, as shown in the figure. While this was in- 

 creasing, so far as we could discover in our earlier studies — in the 

 course of a couple of minutes — the nucleus C showed a definite 

 dividing line, and the " trailing flagellum " also rapidly split, as at h. 



The division first initiated widened (D, Fig. 2, e), and the front 

 flagellum became wholly divided, the trailingfone not splitting so fast; 

 but the nucleus showed, as at d, a strong tendency to fission. 



Very rapidly the bisection ensued, until generally, in not more 

 than four minutes, the division had been wholly effected lengthwise, 

 as in Fig. 3, E, "-, h, and the nucleus was wholly divided save for a 

 fine thread connecting the parts, as at /, K, and the trailing flagellum 

 was nearly divided. 



Soon the protoplasmic neck connecting the dividing forms was 

 almost wholly gone, as in Fig. 4,"F, and the two nuclei, n, 0, were as 

 perfect as in Fig. i. A, that is to say, a highly refractive body with a 

 distinct envelope, or border ; and the two perfect forms, /, m, Fig. 4, 

 become disconnected, dividing the trailing flagellum to its root, and 

 each being free. 



This process is exhaustive in the great majority of the forms. 

 By following successive fissions — that is, one-half of each division — 

 it was found that the end came at from four to five hours. In other 

 words, this process of division ceased by the death of the last-divided 

 form. But this only applied to about eighty-five in every hundred. 

 The remaining fifteen go through a very curious change. 



Fig. 5 represents a final form derived by fission. Its chief 

 distinction is a strong nucleus, and always a free-swimming condition. 



