264 CHROMATIN, ANIMAL CHROMOSOMES, NUCLEOLI 



tissue is, therefore, the more likely one is to obtain good 

 fixation. 



625. General Methods. In animals, material for chromosome 

 studies may be found either in germinal or in somatic tissue. 

 Germ cell divisions, especially meiotic stages, must be sought in 

 the gonads at the proper time in the life cycle, which varies in 

 different groups. Somatic divisions are most numerous in 

 embryos and in larval stages. Among the higher vertebrates the 

 amnion gives excellent chromosome figures and this tissue has been 

 used extensively in mammals having a high chromosome number. 

 Dividing mesenchyme cells give good figures provided the embryo 

 has been cut up so that this tissue is well preserved. Among the 

 invertebrates, especially in insect larva?, the brain and the larger 

 ganglia have been favourite tissues. 



In the past, the standard procedure has been to make paraffin 

 sections of fixed tissue and to stain these in iron hsematoxylin, or 

 some other nuclear dye. Some investigators have used the smear 

 method (see §§ 645, 1126 for details) for spreading the tissue into a 

 thin layer, after which it is fixed and stained by the usual methods. 

 More recently, many cytologists have found that excellent 

 chromosome preparations may be had, if organs, such as the 

 gonads of insect larvae, are killed and stained with aceto-carmine, 

 and then spread out or mashed, by pressure on the coverslip 

 until a thin layer of cells is obtained. This last method is so 

 quick and simple that it should be thoroughly tried out before more 

 elaborate techniques are resorted to (see §§ 257, 634 for details). 



Fixatives. For animal tissues the inexperienced worker will 

 do well to begin with Bouin's fluid or some other picro-formol- 

 acetic combination, because these penetrate evenly and deeply, 

 do not overfix, are cheap and easy to use. The main disadvan- 

 tages of such solutions are, first, some cytoplasmic structures are 

 not fixed well, and, certain nuclear anilin dyes do not take as well 

 after picric acid fixation as they do after the use of osmic acid, and 

 it is necessary to mordant with the latter if these special dyes are 

 to be used (see § 1364). For these reasons, for a well-rounded study 

 of cellular structures picro-formol-acetic fixation should be 

 supplemented by material treated with Flemming's solution, or 

 some one of the other chromo-aceto-osmic mixtures. The latter 

 have the disadvantages, however, of not penetrating well, of 

 overfixing if applied too long, of being expensive, and finally, of 

 requiring a bleaching of the tissue before staining. 



After one has become familiar with a particular type of tissue, 

 it will often be profitable to vary the proportions of the ingredients 

 a little, for experience has shown that even slight variations may 

 have a marked effect both on the density of cell parts and many of 

 the finer details. For example, there are numerous slight 

 modifications of Bouin's, of Flemming's solutions, and of the 



