MITOSIS AND AMITOSIS. 4OI 



If the temperature is a little higher (37 C.) the chromosomes 

 may stick together in an irregular mass and be drawn to the 

 surface of the cell, probably by the complete contraction of the 

 fibers \vhich anchor the spindle to the peripheral layer of the 

 protoplasm (Figs. 13, 14, 17). In the latter instance the chromo- 

 somes do not swell up and become vesicles, but remain perma- 

 nently small and densely chromatic. Owing to some change, 

 probably in the peripheral layer of each chromosome, caused 

 by the high temperature, the chromosomes are unable to take 

 up achromatin. Such chromosomes and the cells containing 

 them never recover and never go further in development, al- 

 though the cells do not immediately undergo degeneration and 

 to all appearances remain alive for twenty-four hours or longer. 



Of a piece with this scattering of chromosomes is the failure 

 of all the chromosomal vesicles to unite into a single vesicle. 

 All degrees of fusion of chromosomal vesicles may be found from 

 those which remain wholly separate to those which are united 

 into a single spherical vesicle. Other things being equal, the 

 size of a nuclear vesicle varies according to the number of chromo- 

 somal vesicles which enter into it. In this way arise nuclear 

 forms which have been called "fragmented nuclei," "multi- 

 nuclear cells," "bipartite," "lobulated" and "elongated" nuclei 

 (Figs. 1-12, 19-28), although it is evident in the case of Crepidula 

 that these nuclear forms have not arisen by constriction or 

 fragmentation of an originally single nucleus. 



These partial nuclei formed by incomplete fusion of the 

 chromosomal vesicles were first called "karyomeres" by Fol. 

 That they are partial and not entire nuclei is shown by the fact 

 first established by Boveri that in the next following mitosis 

 each karyomere gives rise only to the same number of chromo- 

 somes as entered into it and not to the full number of chromo- 

 somes characteristic of the species; this fact I have repeatedly 

 confirmed in my studies on Crepidula. And that such karyo- 

 meres are due to the failure of chromosomes to unite into one 

 vesicle and not to the amitotic division of a single vesicle is 

 shown by the following considerations: 



I. They are most numerous in the telophase of division (Figs. 

 712) where the chromosomes are sometimes widely scattered 



