x TECHNICAL DIRECTIONS 511 



a few millimetres above the blastoderm and the india-rubber bulb 

 squeezed rhythmically so as to wash away the particles of yolk bv 

 very gentle currents of salt solution. When the blastoderm is lifted 

 out of the solution stranded upon the coverslip it is very apt to 

 become folded. When this happens, on account of the fragility of 

 the blastoderm, no attempt should be made to stretch it out by the 

 use of needles or forceps. The folds should rather be straightened 

 out by a current of salt solution allowed to flow out from the orifice 

 of a pipette held vertically just over the centre of the blastoderm. 



IV. EARLY BLASTODERMS. Open the egg as before. Let the 

 albumen run off until the vitelline membrane over the blastoderm is 

 exposed. Raise the egg until the blastoderm touches the surface of 

 the salt solution and then bring a wide-mouthed pipette of Flem- 

 ming's solution, held vertically, into such a position that its tip just 

 touches the film of fluid over the blastoderm. Let the solution flow 

 down slowly on to the vitelline membrane covering the blastoderm. 

 If there is any albumen overlying the blastoderm this should be 

 carefully stripped off as it coagulates. A small piece of black bristle 

 should be stuck into the vitelline membrane on each side to mark 

 the line joining the chalazae so as to facilitate the orientation of the 

 blastoderm for section-cutting. The fixing fluid should be allowed to 

 act for several minutes and then a circle of vitelline membrane 

 may be excised with the blastoderm adhering to it. Float' out the 

 circle of vitelline membrane on a coverslip with the blastoderm above 

 and submerge in a watch-glass of fixing fluid. If the circle of blasto- 

 derm adheres to the coverslip so much the better : it may be separ- 

 ated in the clearing agent. 



Instead of a pipette as above indicated being used for the fixing 

 fluid a small rim of cardboard, e.g. the rim of a small pill-box lid, may 

 be placed on the surface of the yolk, raised up slightly out of the 

 salt solution, so as to enclose the blastoderm and then the little tank 

 so formed may be filled with Flemming's solution which will gradu- 

 ally diffuse downwards. Minchin recommends a triangular instead 

 of a circular rim for facilitating subsequent orientation. 



For fine work it is preferable to embed the whole yolk in celloidin 

 and then after the celloidin has been hardened to cut out the portion 

 in the region of the upper pole for sectioning. This method con- 

 sumes however much more time than does the paraffin method. 



V. THIRD-DAY EGG. A. Open the egg as before. 



B. Study the embryo and blastoderm while still alive and in 

 situ. A large outline drawing should be made. The details of the 

 body of the embryo will be seen better later but the arrangement of 

 the blood-vessels can best be studied now while the circulation is 

 still active. As a rule they can be seen distinctly through the 

 vitelline membrane but if not the latter should be carefully stripped 

 off. A Greenough binocular microscope with its lowest power 

 otyectives is a useful accessory for examining the blood-vessels. 



C. Excise the embryo with the surrounding portion of blasto- 



