BY R. J. TILLYARt). 41 



same time, cause the least deformation of the cell-layers within 

 the gill. 



Fixation : — I find the two best fixatives are the well-known 

 Gilson's Fixative (mercuro-nitric), and Carls' Fixative (Formula: 

 2 parts glacial acetic acid, 6 parts concentrated formol, 15 parts 

 absolute alcohol, and 30 parts distilled water). Both fixatives 

 give equally good fixation of all the internal parts of the gill; but 

 neither of them is quite satisfactory as regards the fixation of the 

 hypoderm-layer, which is often found to be somewhat shrunken. 

 In addition, if Gilson's Fixative be used, it is very difficult to 

 remove all traces of mercuric chloride from the innermost of the 

 alveoli, in the case of large saccoid gills. The remarkable struc- 

 tures described by Ris(28) in some of the alveoli of the saccoid 

 gill of Fseadophaa, in the form of moSvS-like tufts of fibrils, appear 

 to me to have been undoubtedly caused by incomplete washing- 

 out of some mercuric or other fixative. I have never found any 

 structures remotely resembling these in any sections that I have 

 examined. 



The fixative may be employed either hot or cold. The former 

 gives the quicker fixation, but has the disadvantage of generally 

 causing the hypoderm-layer to shrink away from the cuticle. The 

 cold fixative, too, can be carried into the field, and larva; fixed on 

 the spot when necessary. 



The gills may be cut ofi' and fixed at any stage of lar\ al growth; 

 but really good sections can only be surely obtained if the larva 

 is taken fairly soon after an ecdysis, when the cuticle is still 

 moderately soft. For this reason, all larxa^ dredged out from 

 ponds or rivers while in this condition should be fixed on the 

 spot, since such larva? do not carry well; and, e^■en ^if they can 

 be kept alive, the delay in returning home means a corresponding 

 increase in the hardness of the cuticle. Unsuitable larv?e should 

 be left in the aquarium until they undergo an ecdysis, when 

 they should be immediately taken and fixed. 



In all cases, fixation should be carried out by cutting the gills 

 ofi" the living larvae, and allowing them to fall directly into the 

 fixative. For transverse sections, it is advisable to make two 



