February. 1911. 



KNOWLEDGE. 



75 



if 



glass with balsam or shellac, it should now be nibbed on the 

 glass plate until the section becomes translucent. During this 

 operation the plate must be well supplied with a paste of 

 carborundum powder and water, using plenty of water. The 

 rubbing is done with a circular motion, and it is advis.ible to 

 make the circles as small as possible, as then there is then less 

 tendency to bevel the edges of the section. Care should be 

 taken also to do the rubbing over the entire surface of the 

 plate, so that it wears evenly. 



If, on the other hand, the rock has been fastened on with 

 gum arable, the rough grinding by hand must be done on the 

 glass plate with dry carborundum powder or otherwise the 

 water will cause the rock to leave the glass. The rough work 

 is done so ([uickly, however, with the grinding lap on the 

 machine, that it can be used 

 whether the rock is cemented 

 with shellac or gum arable, 

 as the grinding is so rapid 

 that the gum has no time to 

 dissolve. When doing the 

 rough grinding on the lap, 

 great care must be taken that 

 the section is ground evenly 

 on all sides, and that the 

 operation is not carried on 

 too long, because the lap is 

 revolving so rapidly that in a 

 few seconds the whole of the 

 rock may be removed from 

 the glass. 



It is diflficult to give 

 directions as to how long 

 this grinding with the coarse 

 powder should be continued, 

 as it depends, in a great 

 measure, upon the rock itself, 

 but a little practical experi- 

 ence will soon decide this for 

 the operator. A rough-and- 

 ready rule is to stop when 

 the rock shows signs of 

 chipping off at the edges. 



It should then be transferred 

 to the other glass plate, with 

 finer carborundum, and the grinding continued by hand until 

 the rock is sufficiently thin. Great care nuist be taken in 

 these later stages, and the rock must be examined under the 

 microscope from time to time to see if it is thin enough. The 

 final rubbing must be done on Water-of-Ayr stone, or one of 

 the very fine carborundum stones used for razor sharpening. 



\\'hen the rock is fixed with gum arable, the final grinding, 

 after the use of the lap, is done on a glass plate with FF 

 carborundum used dry, finished off on FF emery cloth, and 

 polished on No. O emery cloth. As mentioned previously, it is 

 not necessary to get a high polish, and any small scratches are 

 practically unnoticeable when the section is mounted in balsam. 



The next operation is to remove the section from the glass 

 holder. If it is cemented with balsam or shellac, it should 

 be placed in ordinary commercial methylated spirit, and must 

 remain out of reach of dust until it has left the glass. This 

 usually takes some time, but it must not be assisted in anyway 

 by pushing the section with a brush or needle, as this is almost 

 certain to destroy it. 



If it is stuck on with gum. it is simply put in water and 

 usually lea\es the glass in about an hour. In either case 

 after the rock has left the glass, it is rinsed in clean methylated 

 spirit, and is then ready for mounting. 



When moving the sections from one receptacle to another I 

 always use large section lifters about one-and-a-half inches by 

 one inch. These I cut out of thin sheet brass with a pair of 

 scissors. They have practically no handle, and will, therefore, 

 stand where they are placed without falling backwards. I find 

 these very useful when placing sections in the final methylated 

 spirit, as lifter and section are put in the receptacle together. 



The mounting is done in the ordinary way in balsam and 

 benzole. The most difficult part is the transference of the 



\ 



,^ 



Figure 1. 

 Ovipositor and Egg of Hydrachna gcograpJiica X 56. 



section from the lifter to the glass slip. The section when 

 taken from the spirit is pushed with a sable brush carefully 

 over the edge of the lifter until it projects as far as is safe, and 

 is then picked up on the hairs of the brush, and transferred to 

 a heated slide. The heat of the slide soon evaporates the 

 spirit, and then a drop of balsam can be applied to the ed^e of 

 the section, under which it will be drawn. .Arrange the section 

 in position on the slip with a needle, examine under the 

 microscope with polarized' fight to see there are no bits of 

 fluff or other forei.i,n substance, put a drop of balsam on the 

 section and then the cover glass. C. H. C.\f.fyn. 



NOTE OX HYDRACHNA GEOGRAPHICA MULL. 

 — In May last. .Mr. Braithwaite found a female Hydrachna 



gcograpliica Miill, in Epping 

 Forest. This mite is the 

 X largest of all the known water 



/ mites. The female often 



measures as much as eight 

 millimeters long in the body. 

 Mr. Braithwaite, hearing 1 

 was in want of a full>- 

 developed female of this 

 species, kindly sent it on to 

 me. After I had draw'n and 

 measured the mite I dissected 

 out the epimera and genital 

 area, and mounted that part 

 in balsam for future reference 

 and for comparison with 

 other species of the same 

 genus. I did not know 

 before, but while I was ex- 

 amining it during mounting I 

 found the o\ipositor was fully 

 extended and that an egg was 

 in the act of leaving the tube, 

 and I am pleased to say this 

 position has been retained, 

 even now it is permanently 

 mounted in balsam. I have 

 mounted hundreds of genital 

 areas of water mites, but 

 never had the good fortune 

 to find one like this before, and I do not expect to 

 ever find another. I mounted it without pressure, 

 placing the cover glass on the liquid balsam and allow- 

 ing it to settle down by its own weight. No doubt the 

 slightest pressure would have squeezed the egg out of its 

 position. The length of the genital plates with the ovipositor 

 is 1-04 mm., length of egg 0-24 mm. The drawing was made 

 under the camera-lucida, with one-inch objective. It shows 

 the exact position of the genital area, and its position between 

 the third and fourth pair of epimera. The female was full of 

 ova. but the eggs in the body were spherical, so the oval form 

 taken by the egg shown in the figure is no doubt due to it 

 being squeezed through the ovipositor. 



Ch.\s. D. So.\r. F.R.M.S. 



THE DIVISION OF THE COLLAR-CELLS 

 IN CLATHRIXA CORIACEA.— In the \o\embernmnber 

 of the Quarterly Journal of Microscopical Science (Vol. 55, 

 p. 611) a contribution is made under the above title, by Miss 

 M. Robertson and Professor E. A. Minchin, to the theory of 

 the centrosome and blepharoplast. It is now generally 

 admitted that these bodies are of essentially the same nature. 

 The centrosome may be briefly characterised as a bod\' which 

 e.xerts or governs kinetic functions in relation to division of 

 the nucleus, while the latter, or blepharoplast, may be defined 

 as a centrosome which governs motile organs, such as flagella, 

 which arise from it. and are in more or less direct connexion 

 with it. The results are briefly simimarised by the authors at 

 the end of the paper. The nucleus of the collar-cell migrates 

 from the base to the apex of the cell, and so comes to he 

 immediately under the blepharoplast. The flagellum dis- 

 appears, and the blepharoplast di\ ides. The two daughter- 



