Fixing 



WHOLEMOUNTS IN RESINOUS MEDIA 



53 



cient 40% formaldeh3^cle to bring the 

 total strengtli to 4%. These forms may 

 then be transferred to alcohol for staining 

 and preparation as resinous wholemounts 

 or may be mounted directly in formalde- 

 hyde as described in Chapter 2. 



Stalked Ciliate Protozoans. These 

 forms are quite easy to fix provided one 

 reahzes that double narcotization is neces- 

 sary: once for the stalk and once for the 

 head. The author's technique is to nar- 

 cotize with Rousselet's solution until the 

 snapping movements have slowed up and 

 then very gently to add weak hydrogen 

 peroxide. 



The specimens are then watched under 

 a microscope and the selected fixative — 

 which must contain osmic acid — is flooded 

 onto them at the exact moment when the 

 cilia straighten out and become stationary. 

 This is satisfactory' with Carchesium, Zo- 

 othamnium and Vorticella. Opercularia and 

 Epistylis have noncontractile stalks and 

 one need, therefore, only use hj^drogen 

 peroxide. The writer has never made a 

 satisfactory mount of Scalhidium or 

 Pyxicola. 



CoELENTERATA. Hj'droids are usuallj' 

 narcotized with menthol, though the 

 writer prefers liis own mixture (Chapter 

 19, AF 51.1 Graj') for the purpose, and 

 fixed in a hot mercuric-acetic mixture. A 

 description of the narcotization of Hydra 

 is given in Chapter 9 and a detailed ac- 

 count of the preparation of Medusae in 

 Chapter 20. Anthozoa, particularly the 

 small ones likely to be prepared as whole- 

 mounts, can be narcotized with menthol, 

 though magnesium sulfate is better. 



Platyhelminthes. Some of the smaller 

 fresh water Turbellaria (e.g. Vortex, 

 Microstortmm) may be narcotized satis- 

 factorily by adding small quantities of 2% 

 chloral hydrate to the water in which they 

 are swimming. Another good techniciue is 

 to isolate the forms in a watch glass of 

 water and place the watch glass under a 

 bell jar together with a small beaker of 

 ether. The ether vapor dissolves in the 

 water and narcotizes these forms excel- 

 lently. A detailed account of the method of 

 handling the liver fluke is given in Chapter 

 20 and may be satisfactorily employed 

 for other parasitic flatworms. 



Annelida. Small, marine, free-living 

 Polychaetae make excellent wholemounts 

 and do not usually need to be narcotized 

 before killing. They should, however, be 

 stranded on a slide and a very small 

 quantity of the fixative dropped on them, 

 so that they die in a flat condition which 

 makes subsequent mounting possible. 

 Much more reaUstic mounts are obtained 

 by this means than if they are laboriously 

 straightened before fixing, for they usu- 

 ally contract into the sinuous wave which 

 they show when swimming. There seems 

 to be no certain method of fixing the 

 Nereids with their jaws protruding and 

 one has to rely on chance to obtain one in 

 this condition. The free-swimming larvae 

 of marine polychaetes are very difficult to 

 fix fsatisfactorily, because the large fiota- 

 tion chaetae usually fall out. The writer 

 prefers for these, as for other marine 

 invertebrate larvae, to concentrate a 

 relatively large quantity of the plankton 

 and then to flood over it three or four 

 times its volume of Bouin's fixative 

 (Chapter 18, F 5000.1010 Bouin 1897) at 

 70°C. The specimens are then allowed to 

 settle, the fixative poured off, and re- 

 placed with 70% alcohol which is replaced 

 daily until it ceases to extract yellow 

 from the specimens. By hunting through a 

 large mass of plankton so fixed, one can 

 usually obtain a considerable number 

 of specimens in a perfectly expanded 

 condition. 



Fresh water Ohgochaetes are best 

 narcotized with chloroform, either by 

 adding small quantities of a saturated 

 solution of chloroform in water, or by 

 placing them in a small quantity of water 

 under a bell jar in which an atmosphere of 

 chloroform vapor is maintained. Leeches 

 are difficult to handle and the author 

 has had most success bj' placing them 

 in a fairly large quantity of water to 

 which is added, from time to time, small 

 quantities of a saturated solution of 

 magnesium sulfate. As soon as the leeches 

 have fallen to the bottom considerably 

 larger quantities of magnesium sulfate 

 can be added, which will leave the leeches, 

 in a short time, in a perfectly relaxed, 

 but not expanded, condition. They should 

 then be flattened between two sUdes and 



