CELLS IN DIVISION 



sure that they never otherwise can become lobulated or dumb-bell 

 shaped. This is certainly not true of tissue cultures, for living nuclei 

 can remain in such forms for many hours. 



The clearest instances of the formation of binucleate cells by 

 amitosis are seen in glandular tissues of Arthropods where the cells 

 of the adult organ become binucleate at a time long past that when 

 normal mitosis has ceased. Kater^^^ gives as examples of this the 

 hepato-pancreas of the Crustacean Porcellio, and the Malpighian 

 tubules of the cricket. Amitotic division in these tubules in the walking- 

 stick insect [Diapheromera) is described by Marshall.^ ^^ 



It has been argued that amitosis must occur in some growing tissues 

 because normal mitotic division is insufficient to account for the 

 observed rate of cell multiplication. Stough^^^ computed mitotic 

 indices in the 1-3 day chick embryo, and calculated the expected 

 increase in the number of cells in the embryo after a given time. His 

 reasoning suggested that the rate of cell multiplication by mitosis was 

 inadequate to account for the cellular growth of the embryo, and so a 

 'cryptomitotic' modification of normal nuclear division described in a 

 previous paper (Stough^^^) was called upon to make up the deficit. 

 The main difficulty in counting mitoses in tissues such as those of the 

 embryo chick is to judge between nuclei in interphase and in early 

 prophase. Mainly for this reason, mitotic counts of different observers 

 vary; those of Stough are consistently well below those .ofScHULTz^^^ 

 for the same tissue and time of development. It is thus probable that a 

 mitotic time of one hour which Stough used in his calculations is not 

 relevant to his data. He suggests that a shorter time of 27-37 minutes 

 would be consistent with his calculations, but rejects such a figure for 

 the duration of mitosis; yet it would probably be more appropriate 

 for the purpose.* A more incisive example of this argument for direct 

 nuclear division is that of the adult mammalian liver, where mitoses 

 are said either to be absent (Munzer^^^) or very rare (Brues and 

 Marble^^'^). Yet the organ grows by increase in cell number, and the 

 frequency of binucleate cells increases (Clara^^^) . Wilson and Leduc^^® 

 are of the opinion that the mitotic process in the liver cell is gradually 

 slurred over with advancing age of the tissue. In the young animal 

 there are full and typical mitosis; later binucleate cells are formed 

 by the suppression of cytoplasmic cleavage. Next in this series, large 

 nuclei arise by an endomitotic process, and finally binucleate cells are 

 formed in a cryptototic or amitotic manner. MacMahon*"^ considers 

 that mitosis and amitosis are interchangeable in the liver. 



* Woodard'^* has criticized Stough's reasoning along the same Hnes. He considers that 

 the intermitotic period in the chick embryo may be more constant than the duration of 

 mitosis. Stough's photomicrographs of sections of his material suggest that some ol his 'modi- 

 fied mitoses' may be degenerating nuclei, which are often found in embryonic tissues 

 (Glucksmann^**) . 



L 153 



