178 



Andrews: Removing avian blastoderius. 



XXI, -2. 



fixative and the lower end drawn to a point and bent up to 

 be readily used in perforating and then in lifting- up the vitelline 

 membrane. 



Part of the shell of the egg" being removed to expose the 

 blastoderm the pipette is tilled with the fixing liquid aud its tip is 

 thrust throngh the vitelline membrane, external to the blastoderm 

 but not far from it. The pipette is then puUed up so that it teuds 

 to raise up the vitelline membrane and to form a 

 Space between the yolk and the vitelline membrane. 

 Into this Space, as it forms, the fixative is injected 

 by pressure upon the bulb of the pipette. P>y pro- 

 perly directing the curreut it is made to spread out 

 between the vitilline membrane and the blastoderm. 

 The blastoderm soon begins to become opaque 

 and to be less soft. Now the current may be di- 

 rected under the blastoderm to wash it free from 

 the yolk, tili it tloats as a loose cake in the fixing 

 fiuid. It raust then be removed, at once, to a dish of 

 fixative. This may be done by cutting through tlie 

 vitelline membrane and drawing out the blastoderm 

 in a very large pipette or eise by lifting up the 

 blastoderm with a section - lifter. The blastoderm 

 should be flat and free from yolk. Any superfluous 

 yolk should be washed otf as the blastoderm lies 

 upside down in the fixative, using a strong but venj 

 f'nic jet of fixative from a special pipette. 



To obtain good results the Operation must be 

 performed quickly and two possible difiiculties guar- 

 ded against. The first is the entrance of air bub- 

 bles : this is obviated by squeezing out a slow stream 

 of the fixative as the point of the pipette pierces the 

 vitelline membrane. If, however, air does get in between blasto- 

 derm and vitelline membrane it may be sucked out : and if it does 

 not rise to the top of the pipette the pipette may be withdrawn, 

 the air expelled , and the Operation repeated tili successful. The 

 second ditficulty, the two great hardening and resulting brittleness 

 of the blastoderm , may be obviated by removing the blastoderm 

 at the right stage, which is a matter easily learned. 



Such fixatives as corrosive and osmic mixtures give trouble by 

 too rapid fixation and when it is necessary to employ them it is 



