i8 93 . SOME USEFUL METHODS IN MICROSCOPY. 117 



follows. The blastoderm is placed in a watch-glass containing dis- 

 tilled water, in which a coverslip is submerged. The coverslip, of 

 course, only touches the watch-glass with its four corners, and there 

 is a space underneath it, between it and the bottom of the watch-glass. 

 The blastoderm is now carefully floated on to the middle of the cover- 

 slip, so that its primitively external or upper surface is downwards 

 and touching the glass. Then with a syringe or pipette the water is care- 

 fully drawn off till it falls below the level of the coverslip, so that the 

 blastoderm is left stranded on the coverslip. As soon as this has been 

 effected the coverslip can be lifted up with a forceps with the blasto- 

 derm upon it. Theblastoderm must not be allowed to dry completely up, 

 but all the superfluous moisture must be drawn off, so that it is pressed 

 closely to the coverslip by capillary attraction. Its cells will then, in 

 most cases, stick very tightly to the coverslip, which is now turned over 

 and transferred to a second watch-glass containing 30 per cent. 

 alcohol, in which it is placed, with the blastoderm downwards. It not 

 unfrequently, however, happens that the blastoderm becomes unstuck 

 and flies off with great rapidity when it comes into the 30 per cent, 

 spirit. When this occurs the same performance must be gone 

 through, in 30 per cent, alcohol, as has just been done in distilled 

 water, and the coverslip, with the blastoderm stuck on to it again, is 

 removed into 50 per cent, spirit, of course with the blastoderm down- 

 wards. It very seldom comes unstuck a second time. The cover- 

 slip, with the attached blastoderm, can now be transferred with the 

 greatest ease from one liquid to another, and stained, if necessary. 

 Finally, it is brought into absolute alcohol, and then into a watch- 

 glass of oil of cloves, without, however, there being sufficient oil of 

 cloves to immerse the coverslip, but only enough to wet the side to 

 which the blastoderm is attached. The coverslip is then placed on 

 a slide, after being supported with wax feet at its corners to prevent 

 the blastoderm being crushed, and mounted in Canada balsam. 



In this way most admirable preparations of blastoderms can be 

 obtained, which not only show cell-structure and nuclear division to 

 perfection, but also are all that could be desired from the morpho- 

 logical point of view ; the radiating cells, for instance, which, at a late 

 stage of segmentation, spread out from the edge of the blastoderm over 

 the yolk, being most perfectly preserved. I have satisfied myself, by 

 careful examination, that if the method is carefully carried out, no 

 cells are left behind on the yolk. The method of sticking the blasto- 

 derm to the coverslip gives the further advantage that, while the 

 blastoderm can be effectually protected from being crushed when 

 mounted on the slide, it is only separated by the thickness of the 

 coverslip from the objective, and can therefore be studied with any 

 power that is desired, however high. 



If, on the other hand, it is desired to cut into sections the blasto- 

 derm so removed, the process is just the same as above, with the 

 difference that a thin slice of liver is substituted for the coverslip. I 



