CHROMATIN, ANIMAL CHROMOSOMES, NUCLEOLI 271 



If aceto-carmine preparations are desired, the ovaries are placed 

 on a clean slide with a pipette, the dissecting medium quickly 

 removed (avoid any evaporation) and iron-aceto-carmine is 

 added from one side in considerable excess. (Adding the stain 

 in this way usually causes the ovaries to stick to the slide and 

 makes the subsequent handling of the material easier.) The 

 ovaries are stained until they are a dark red, which will take from 

 ten to twenty minutes, and then the coverslip is added. A little 

 evaporation of the stain during the process seems desirable. The 

 excess of stain is removed with a pipette and filter paper and then 

 the preparation is blotted with a good deal of pressure. The 

 oocyte divisions are at the small end of the egg strings, so the 

 ovaries should be mashed out by pressing on the coverslip with a 

 blunt needle. The coverslip is sealed with vaseline, melted 

 paraffin, or in some other way. The preparation is now ready for 

 study, and if artificial light is used, it is well to filter it through a 

 blue-green filter, or a yellow-green filter. Such a slide is best a 

 few hours after staining and will continue useful for several days 

 depending on climatic conditions. 



If sections are desired, the ovaries are transferred from the 

 dissecting medium to the fixative, using a pipette. Strong 

 Flemming's solution is often employed, but it should not be 

 allowed to act more than thirty minutes, less is probably better, 

 and then the hardening should be completed in some other medium. 

 Bridges (personal communication) uses a 1 per cent, solution of 

 chromic acid in which the ovaries are left for a few hours. Painter 

 transfers the ovaries to Hermann's fluid, in which they are left 

 two or three hours {Zeitsch. ind. Abs. Verer., Ixii, 1932, p. 316). 

 After fixation the ovaries are washed for four or more hours. 

 Then they should be packed into pupa cases, which enables one to 

 carry them through dehydrating and imbedding without trouble. 

 (See § 157 for directions for handling small objects). The reason 

 ovaries are left in the Flemming's solution such a short time is that 

 longer fixation causes the cytoplasm of the cells to take on a grey 

 tone, when iroVi haematoxylin is applied. In the experience of 

 the writer, Bouin's fluid, or Bouin-Allen, causes the cliromosomes 

 to contract much more than they do in osmic fixatives. 



If desired, smears may be made of ovaries in the usual way 

 (see § 625). 



637. Spermatogonia! and Meiotic Stages. These must be sought 

 in very young larva?. A larva, reared at ordinary room tempera- 

 tures, will show all stages up to spermatids, when it is four days 

 old. Spermatogonial divisions may be found occasionally in the 

 adult testes. Any of the standard fixatives can be used. (At this 

 early age one cannot distinguish between male and female larvae, 

 but when one dissects out the gonads, the testes will be found 

 to be considerably larger than ovaries in larva- of the same size.) 



