CHAPTER XXVI. 305 



In the same way more fluid chromosomes such as those of mammals 

 may be caused to run together arid so introduce error. This probably 

 explains the discordance in the accounts given by different workers for 

 such material as that of man. 



655. Note on Fixatives for Chromosome Work. For work on 

 vertebrate tissues there is little doubt that Bouin's picro-formol- 

 acetic, or one of its modifications, is the best mixture to use. For 

 invertebrates in general such Bouin fixatives are also very satis- 

 factory, but Flemming's strong formula should always be tried. 

 Corrosive acetic acid I hold to be a rough and unreliable fixative, 

 and if a corrosive fixation is desired, it is better to use Gilson or 

 Petrunkewitsch these penetrate more rapidly and give a more 

 delicate fixation. I have seen some excellent chromosome plates 

 in mammals got by using Sansom's Carnoy modification ( 86). It 

 should be noted carefully that many workers use chromosome 

 fixatives for a short time only ; for instance, Flemming may be used 

 on insect gonads for half to one hour only, followed by a washing 

 out under tap, and then upgrading from 30 per cent, alcohol (from 

 Professor LEONARD DONG ASTER, in literis). 



656. Urea and Chromosome Fixation. It has been claimed by 

 some American workers (McClung, Ezra Allen, K. T. Hance, etc.) 

 that the addition of from 0'5 to 2 per cent, urea crystals to fixatives 

 of the Flemming or Bouin type assists in penetration and gives 

 sharper pictures of the chromosomes. The idea of using urea is, I 

 understand, due to Professor McClung. 



It is out of the province of this book to discuss whether a solution of 

 urea in such complicated fluids as Bouin or Flemming, has the same 

 peculiar penetrative properties as in water, or even to question whether, 

 after the addition of the crystals to the Bouin or Flemming, disintegra- 

 tion of the urea does not take place. We are prepared to accept the 

 statements of McClung, Allen and Hance, and to recommend a trial of 

 the method. WARO NAKAHARA (Jour. Morph., 1919), working on Perla, 

 did not find that the addition of urea crystals to his fixative made any 

 appreciable difference. 



657. Fixation of Mammalian Chromosomes. The material must 

 be absolutely fresh ; even half an hour's delay is fatal ; prepare your 

 fixatives and capsules, knives, etc., before you kill the animal. 

 Apparently chromosomes of mammals will clump together about 

 ten minutes after death. HANCE (Anat. Record, xii, 1917) gives the 

 following method : (1) Obtain fresh specimens of tissue from as 

 many different animals as possible, so as to be sure of obtaining one 

 or more in a " cycle of cell division." (2) Place small or finely 



M. 20 



