THE REPRODUCTION OF THE PROTOZOA 



113 



FIG. GO. Details of the structural changes of the nucleus of Euglypha alvedata 

 during karyokinesis, showing the formation of the chromosomes. A, 

 Coarsely - meshed condition of the nucleus ; the chromatin-granules 

 aggregated at the nodes of the reticulum. B, Later stage ; the nucleus 

 beginning to show a fibrous structure as a result of the irregular clumps 

 of chromatin-granules of the previous stage becoming arranged in linear 

 series. B 2 , Some of the fibrils of this stage more highly magnified. 

 0, Later stage ; the fibrils have become smoother and more parallel in 

 arrangement. C 2 , Fibrils more highly magnified ; they consist, as in the last 

 stage of darker and lighter parts (the former chromatin, the latter probably 

 plastin) ; between the individual fibrils are cross-connections, more regular 

 in this stage than in the last (remains of the nuclear reticulum). D, The 

 fibrils have become shorter and thicker, and are bending up to form the 

 U-shaped chromosomes. After Schewiakoff (100) ; magnification of A, B, 

 C, and D, about 1,200 diameters. 



of ciliate Infusoria, such as Paramecium. The macronucleus divides 

 without mitosis, as stated already, but the micronucleus exhibits a 

 primitive type of mitosis (Fig. 61). When division begins, the 



FIG. 59 continued: 



D, About fifteen minutes later than G. The daughter-shell is now com- 

 pletely formed, and the middle granular zone of the parent is passing over 

 into it ; the nucleus of the parent has assumed its definitive orientation, with 

 the centrosomes at the poles of an axis coincident with the longitudinal axis 

 of the animal, and the equatorial plate is definitely formed. 



E, About thirty minutes later than D. The whole of the middle zone of 

 the parent has passed over into the daughter-shell ; the flattened form of the 

 nucleus is changing into an elongated spindle-form, and the equatorial plate 

 is splitting to form the two daughter- plates. 



F, About five minutes later than E. The daughter-plates have travelled 

 apart, and the division of the nucleus is beginning. 



G, About five minutes later than F. The division of the nucleus is com- 

 plete, and one daughter-nucleus has passed over into the body of the daughter- 

 Eiiglypha. 



H, About twenty-five minutes later than G (about 125 minutes from the 

 beginning). Some of the protoplasm of the middle zone flows back into 

 the parent-shell, and each individual has its own contractile vacuole ; the two 

 daughter-nuclei are reconstituted, and the karyosome has reappeared in 

 each ; pseudopodia are being protruded from the mouths of the shells ; the 

 division is complete, and the animals are beginning to separate. 



After Schewiakoff (100) ; magnification about 470 diameters. 



8 



