THE devi*;lofment of LEPIDOSIRKN PARADOXA. 5 



during the embedding process I use a special apparatus^ ia 

 which a pool of paraffin in contact with the block holder of 

 the microtome is kept melted by a small loop of platinum, 

 nickel, or other wire of high resistance and not easily 

 oxidisable, heated by the current from one or two ordinary 

 bichromate cells. 



Staining. — After many trials of different staining fluids 

 I adopted two stock methods. 



T. Early eggs rich in yolk were stained in Griibler's 

 " Safranin 0" — a saturated solution in absolute alcohol, 

 diluted with an equal volume of distilled water. In regard 

 to formalin eggs, difficulty was found in obtaining a good 

 chromatin differential stain. This difficulty was completely 

 got over by treating the eggs with corrosive sublimate 

 solution for a couple of hours before transference to alcohol. 

 II. Later embryos were stained in Heidenhain's iron 

 htematoxylin followed by faint staining with eosin. By this 

 stain beautiful preparations Avere obtained showing minute 

 nuclear detail to perfection. 



Mounting Medium. — When sections of early eggs did 

 not stain successfully they were mounted in colophoniura, 

 which on account of its lower refractive index shows up 

 feebly stained structures better than Canada balsam. 



Reconstruction. — In working out the organogeny of 

 Lepidosiren I have found the following method of recon- 

 struction from serial sections extremely useful. Sections 10 ^u 

 thick are drawn with the Abbe camera lucida at a magnifi- 

 cation of 100 diameters upon finely ground sheets of glass 

 1 mm. in thickness. Sheets of glass bearing drawings of 

 consecutive sections are then piled in position on top of one 

 another, a fluid of the same refractive index as the glass 

 being run in between adjacent sheets. The result of this 

 is to convert the whole into a transparent block, in which 

 the structures drawn are seen occupying space of three 

 dimensions, forming a kind of model. Different organs are 

 drawn in different colours, lead pencil or coloured crayons 

 ' Made for me by tlie Cambridge Scientific Instrument Company. 



