Coral 



GROUND SECTIONS 



85 



the mounting of these l)one sections. Some 

 people prefer to mount them dry, in which 

 case tlie shde is pkiced in a jar of benzene 

 and left until all the Canada balsam has 

 been removed. Each individual section is 

 then picked up on a section lifter, trans- 

 ferred to two or three fresh changes of 

 benzene to remove the last of the balsam, 

 and then dried under pressure between 

 two glass slides; when it is dry it is then 

 treated as any other dry wholeniount (see 

 Chapter 1). This method undoubtedly 

 makes it easier to see the finer structure of 

 the bone, but it is applicable only to very 

 thin sections, for the additional transpar- 

 ency imparted b}' the balsam will be lost. 

 It is usually more satisfactory to melt the 

 balsam, to lift each section up, to place it 



in fresh balsam on its individual slide un- 

 der a coverslip, and to heat it until all 

 the air bubbles have been expelled. The 

 coverslip is then held down with one of the 

 clips shown in Fig. 25 and cooled. 



All these operations can be conducted 

 much more conveniently on the machinery 

 made for grinding and poUshing rock sec- 

 tions, but this description has been given 

 for the benefit of those who lack such 

 machinery. It has from time to time been 

 recommended in the literature that one 

 should grind sections of this kind down on 

 microtome sharpening stones, using oil as 

 a lubricant. The writer has had the most 

 wretched results by this method ; it is men- 

 tioned only in order that it may be 

 avoided. 



Preparation op a Transverse Section of a Coral with Polyp in Place 



It must be understood, first of all, that 

 the preparation here to be described will 

 give a very much less satisfactory trans- 

 verse section of a coral polyp than will 

 the paraffin method described in Chapter 

 12. This method is intended onl}^ as a 

 compromise between a paraffin section of 

 a polyp and a ground section of a hard 

 structure of the type described in the last 

 example. 



The living animal must first be narco- 

 tized, fixed, and hardened. It does not 

 matter what coral be selected. The north- 

 ern coral {Astrangia danae) is convenient 

 both because of its wide distribution and 

 because it ma}- be obtained from biological 

 supply houses. Supposing, however, it is to 

 be collected fresh, a piece should be secured 

 of about the size of an orange, or smaller, 

 and brought back to the laboratory and 

 left to expand in plenty of well-aerated sea 

 water. It must, of course, be narcotized 

 before fixation and a double process is best 

 for this type of specimen. About 5% of its 

 volume of a saturated solution of mag- 

 nesium sulfate is therefore added to the 

 water and the action of this narcotic is 

 enhanced by sprinkhng menthol on the 

 surface. After about half an hour the pol- 

 yps will be extruded from the coral in a 

 partially narcotized condition and should 

 be fixed. Perfect narcosis will result in such 

 a small quantity of the polj'p remaining in 

 the coral that it is scarcely worth while 



sectioning it, whereas fixing an unnarco- 

 tized coral causes such a contraction of 

 the polyp that the sections will be hard to 

 interpret. 



The fixative selected should be one 

 which will harden the poh'p as much as 

 possible without having any effect on the 

 calcareous structure surrounding it. The 

 copper sulfate-mercuric chloride of Lo Bi- 

 anco (Chapter 18, F 3400.0000 Lo Bianco 

 1890) is excellenth' suited for the purpose 

 and about a gallon will be required for the 

 fixation of a specimen of the size described. 

 As much of the water as possible is now 

 siphoned off from the vessel containing the 

 coral specimen and the fixative added. The 

 fixative should be stirred at intervals for 

 the next two or three days and then the 

 specimen should be transferred to fresh 

 fixative for a period of about another 

 week. The coral is then washed in running 

 water overnight and j^laced in 4 % formal- 

 dehyde, which should be changed daily 

 until the whole of the fixative has been 

 washed out of the specimen. 



A coarse hacksaw is then used to cut the 

 specimen into cubes of about an inch on 

 a side, with due regard to the selection of 

 pieces which will subsequently give good 

 sections. These inch cubes are now washed 

 in running water overnight, to get rid of 

 the formaldehyde, and suspended in a con- 

 siderable volume of 70% alcohol. It will be 

 necessary to impregnate them with a resin ; 



