MITOSIS AND AMITOSIS. 411 



absorb substance from the cytoplasm in their growth, but in 

 this case the entire nucleus has undergone constriction and the 

 cell body has not. 



The aggregation of chromatin on the side of the nuclear vesicle 

 on which the spindle lies causes it to collect on the animal pole 

 side of the nucleus in Figs. 49-51 and on the side away from the 

 animal pole in cell D, Fig. 54. In every instance the chromatin 

 collects at that pole of the nucleus which is next to the centro- 

 some or spindle; this is a general phenomenon which has been 

 remarked by R. Hertwig in Actinospherium, by Calkins in 

 Noctiluca, Conklin in Crepidula, etc. In this connection one 

 recalls that in many protozoa the nuclear membrane persists 

 throughout mitosis the spindle being within the nuclear vesicle. 

 The chromosomes divide and separate as in typical mitosis but 

 the nuclear membrane and vesicle constrict as in amitosis. In 

 the ciliata the micronucleus divides by mitosis, the macronucleus 

 by amitosis. In metazoa also the chromosomes alone divide 

 by mitosis, or the splitting of the chromatic thread, while the 

 division of all other nuclear constituents is a mass division. 



The peculiar form of nuclear division which is caused by the 

 stretching out of the chromosomes and the persistence of the 

 nuclear membrane superficially resembles amitosis but is really a 

 modified form of mitosis. Nuclei which have divided in this 

 peculiar manner go on dividing by mitosis when they are returned 

 to normal sea-water. Thus cells A and C, Figs. 49 and 50, 

 cells A and B, Figs. 52 and 53, and all the macromeres in Fig. 54 

 are shown dividing by mitosis. These are typical mitotic figures 

 though the number and arrangement of chromosomes may be 

 atypical. In all of these cases the dividing nucleus of the 

 macromere is connected with the nucleus of the micromere by a 

 chromatic strand and it is particularly noticeable that this strand 

 always runs to the plate of chromosomes in the macromeres and 

 usually to the outer side of this plate. Since the original chro- 

 matic connection united the nuclei by their distal poles (away 

 from the centrosome) the fact that when these nuclei divide the 

 connection runs to the proximal pole (toward the centrosome) 

 indicates that the chromatin changes position within the nuclear 

 vesicle, being drawn to the proximal pole of the nucleus (Fig. 

 54, 



