270 Colchicine 



tion itself. It has also been suggested that the apparent increase in 

 neural tissue was merely the consequence of abnormal cellular migra- 

 tions, not of modified mitotic activity. ^'^ 



Analysis of patterns of embryonic growth is made difficult by 

 many facts. One is the varying sensitivity of tissues and stages of 

 development. In Molge palmata Schneid., the zones of highest mitotic 

 activity are the most sensitive to colchicine;^° in other regions, the 

 same concentration may yet enable mitosis to recover and to proceed 

 to telophase through star and incomj^lete star metaphase. In Dis- 

 coglossus pictus Orth., some periods of growth are very sensitive to 

 the mitotic arresting activity of colchicine. The fifth day, correspond- 

 ing to the "primary metamorphosis," when swimming is initiated, is 

 one of these periods. In Discoglossis, Rcma, and Xenopus. the meta- 

 morphosis is a period of increased sensitivity. The regions of the 

 embryos where the mitoses are the most numerous are, rather natu- 

 rally, the most rapidly altered by colchicine. Instances are the nervous 

 system, the olfactory bud, and the germinative region of the eyes.^^ 



These carefully studied facts do not leave much to say about 

 papers which attempted to detect zones of growth by colchicine, 

 especially in amphibia, i"^- -^ for the complexities of the problem were 

 not properly understood at the time of their publication. Some facts 

 emerge, however, from the literature on this subject and are worth 

 mentioning, for they may be starting points for further work. In 

 young mice, colchicine demonstrated that liver and pancreatic cells 

 cease to divide at about 20 days after birth;^! the mechanism which 

 prevents any further division, except in regeneration (Chapter 9) , 

 is unknown. In mice also, ganglionic nerve cells have been found, by 

 the use of colchicine, to divide until three weeks after birth.-" Colchi- 

 cine has also been used to bring about the death of the litter of preg- 

 nant mice,!^ and to induce the formation of tetra- and octoploid cells 

 in embryos of the fish Coregonus when the eggs had been treated 

 three hours with a 0.5 per cent solution. Hastening of the meta- 

 morphosis of Rana fiisca tadpoles is also reported. '^^ 



The publications which have been reviewed in the last paragraph 

 would seem to indicate that colchicine is of little, if any, use in the 

 study of embryonic growth. However, it must be recalled that most 

 of these results have been published during the early phases of colchi- 

 cine research, before the proper techniques could have been designed. 

 Two recent papers show that important facts can be made clear by 

 using colchicine as a tool in embryos.^ 



In the first one, the jiroblem was to assess the comparative mitotic 

 activities of the embryonic megaloblasts (young red blood cells) of 

 the chick embryo, and of the megaloblasts of human Addison-Biermer 

 anemia (cf. C^hapter 9) . These cells resemble closely the embryonic 

 ones, though their existence is an evidence of pathological growth 



