app. METHODS OF EMBKYOLOGICAL EESEAEOH 579 



To obtain thinner sections it is necessary to embed the celloidin block 

 containing the object in paraffin. This may be done simply by transfer- 

 ring the block saturated with cedar oil to melted paraffin. A better 

 method is to use a solution of celloidin in clove oil of about the 

 consistency of treacle. The object, thoroughly permeated by this and 

 surrounded by a small quantity of the celloidin, is hardened and cleared 

 in chloroform. The block is then carefully trimmed with one face 

 accurately parallel to the plane of the required sections. It is now 

 immersed in melted paraffin for a minimum time (ten minutes suffices 

 for a small object). After cutting and mounting the sections the slide is 

 immersed in xylol until the paraffin is dissolved out, then in absolute 

 alcohol, then in a mixture of equal parts of absolute alcohol and ether 

 until the celloidin is removed. The slide is now taken down through the 

 series of alcohols and the sections stained and mounted in the ordinary 

 way. 



The arriving at a clear idea of the structure of an embryo from the 

 study of a series of sections involves fitting the successive sections together 

 into a continuous whole. To a great extent this reconstruction of the 

 whole from the successive sections can be done mentally but where 

 complicated structures are being investigated, some aid is either absolutely 

 necessary or at least desirable for the sake of accuracy. The present 

 writer finds the most reliable as well as the most convenient of such aids 

 in the method of reconstruction by means of glass plates. 1 Successive 

 sections are drawn with a hard (9 H) lead pencil by means of a camera 

 lucida upon finely ground sheets of glass such as is used for photographic 

 focusing screens and then the successive drawings are fitted together, a 

 fluid of as nearly as possible the refractive index of the glass being inter- 

 posed between them so that the ground surfaces disappear and the heap of 

 plates appears as a clear block with the structures drawn running through 

 it and appearing as a kind of solid model. 



The following details may be noted. Sections are cut to a standard 

 thickness of 10 /x {i.e. y^-g- mm.) : the glass plates are 1 mm. thick : the 

 drawings are made at a magnification of 100 diameters. But it will be 

 found in practice that much use can be made of the method even if these 

 three dimensions are not so exactly correlated. The outlines made with 

 pencil of the particular organ that is being studied are filled in with water 

 colour. Vermilion is the most generally useful colour for it retains its 

 opacity and light-reflecting properties to an unusually high degree when 

 submerged in fluid of high refractive index. When the plates are dry , 

 No. 1 is laid, ground side up, on a flat surface — preferably a glass stage 

 with a mirror beneath so that light may be reflected up through it — a few 

 drops of the fluid used, e.g. clove oil or cedar oil or a mixture of fennel 

 oil (two parts) and cedar oil (one part) as recommended by Budgett 2 are 

 placed by a pipette on the centre of the ground surface and then plate 

 No. 2 is lowered gently into position and fitted into its place over plate 

 No. 1. The outlines of the drawings should be made to coincide exactly, 

 and the two plates should be pressed firmly into contact care being taken 

 to avoid interposed air bubbles which act as elastic cushions and prevent 

 the upper plate from settling down into contact with the other. Successive 



1 Quart. Joum. Micr. Sci., xlv, 1902. 

 2 Trans. Zool. Soc. London, xvi, Pt. 7, 1902. 



