6 J. GRAHAM KERR. 



(not anilins) being used. It is best, I find, only to do one 

 oi" two systems of organs at a time, the process being so 

 rapid compared to ordinary modelling by Bern's method 

 that it can easily be repeated if necessary. When I first 

 devised tliis method I used a chemical solution having the 

 exact refractive index of the glass, but latterly I have used 

 ordinary clove oil, which is near enough for practical purposes. 

 With clove oil ordinary water-colour pigments may be used.^ 

 The above method is not meant to give a permanent model 

 of the structures investigated, as does the Born method of 

 reconstruction from sections ; but, on the other hand, it in- 

 volves far less expenditure of time, and is to be strongly 

 recommended for purposes of research. The main principle 

 of the method — the using sheets of glass or other transparent 

 plates on which to draw the consecutive sections — has been 

 used by other workers, e. g. Strasser and Dixon, and more 

 recently by Vosmaer. I have not, however, come across any 

 mention in literature of the two details upon which to my 

 mind the chief beauty of the method rests, viz. the using 

 sheets of ground glass to draw upon, and the subsequent 

 rendering these transparent by an interposed fluid of high 

 refractive index. The first of those details provides a par- 

 ticularly suitable surface upon which to draw ; the second 

 gives a perfect transparency to the mass of superimposed 

 plates, quite unattainable where there are numerous alternat- 

 ing layers of substances differing so much in refractive index 

 as do glass and air. 



Early Development op Lepidosiren. 



Segmentation and Origin of Segmentation Cavity. 

 — A vertical section through a mature egg of Lepidosiren 

 shows that the interior is filled with a mass of yolk granules, 

 the protoplasmic substance between being so small in quan- 

 tity as to be quite invisible. The yolk granules are rounded 

 or occasionally subangular in form. Through the greater 



' Mr. Budgett, wlio has been recently using my method of reconstruction, 

 strongly recommends tlie use of moist water-colours. 



