CHROMATIN, ANIMAL CHROMOSOMES, NUCLEOLI 275 



this slowly with Flemming, where the eggs are left for twenty-four 

 hours {Zeitsch.f. Zellforsch. u. mikr. Anat., xxiv, 1936, p. 85). 



For nematodes, Ascaris megalocephala, for example, oogonia, oocytes 

 and fertilisation stages preserve well with Bouin-Duboseq, Flemming, 

 Zenker, or Petrunkevitch. Cleavage stages require such mixtures as 

 Carnoy's (1 : 1 : 1) Carnoy-Bebrun, or acetic-alcohol. 



644. Mounting Sections between Coverslips. Agar {Quart. 

 Jour. Micr. Sci.. 1911) has devised a method for mounting 

 preparations between coverslips in order that they can be 

 observed on both sides. Use one larger coverslip as if it were a 

 slide. Carriers may be made by stamping out a square in stiff 

 cardboard, or thin metal. See also. C. Cepede, C. R. Soc. Biol., 

 civ, 1913. 



645. Smear Preparations of Gonads. In some cases smear pre- 

 parations of testes especially may provide useful evidence in a research 

 on chromosomes. One may be fortunate enough to find nuclei at the 

 prophase or metaphase of mitosis, with all the chromosomes spread 

 out so as to be counted with ease. In many cases, to study early stages 

 in synapsis for which very rapid penetration is essential, smear pre- 

 parations are a sine qua non. 



Remo^'e the testes ; if it is large take a fragment by a pair of forceps 

 and quickly smear along the length of a dry slide several times, so as 

 to cover as much of the middle part of the slide as possible. If the cells 

 are likely to stick, fix immediately by pouring on some Flemming, 

 Bouin, or Petrunkewitsch. Set aside for a few minutes, wash off in 

 water, upgrade from 30 per cent, alcohol, and leave overnight in 90 per 

 cent, alcohol. Bring back to water, stain in iron haematoxylin, thionin, 

 or gentian violet, etc. See also Goodrich's iodine-Bouin method (under 

 "Protozoa "). Note that smears may be fixed in steam, acetic, osmic, 

 formalin vapour, or stained and fixed simultaneously in Leishmann, 

 acetic Bismarck brown (§ 346), aceto-carmine (§ 257), or such mixtures. 

 Smears of very liquid testes, like those of Lepidoptera, are liable to be 

 washed away if fixative is added too soon. It is probably best to kill 

 the cells in some toxic vapour, then allow them to dry a little and then 

 fix in a liquid. 



Foot and Strobell {Arch. f. Zellf., Bd. xii, 1914) recommended the 

 following : Place testes in drop of acidulated (acetic) " water " on end 

 of slide, and with fine needle (No. IX.) cut from it the area at stage 

 required (previously ascertained by examination of sections). Push 

 this area by point of needle to middle of slide and break up the tissue by 

 gently tapping with the needle (never roughly spread as recommended 

 by some workers). This should be done under dissecting microscope. 



See also Kernschwarz, and " Iron Carmine," § 258. For Bataillon 

 and Koehler's borax-methylen-blue see Comptes Rendus, cxvii, 1893, 

 p. 521. 



NUCLEOLI AND CENTROSOMES 



646. The word nucleolus is generally used to mean any large 

 rounded stainable body in the nucleus, but cytologists recognise 

 that at least two types of substances are included under this name, 

 chromatin and so-called plastin. Chromatin nucleoli are made 

 up of chromatin, principally at least, and ma\^ represent whole 

 chromosomes, such as the sex chromosomes in the auxocytes of 



