178 



Andrews: Removinor avian blastoderuis. 



XXI, '2. 





fixati^e and the lower end drawn to a point and bent u]) to 

 be readily used in perforating and then in lifting- iip tlie vitelline 

 merabrane. 



Part of the sliell of the egg being removed to expose tlie 

 blastoderni the pipette is fiUed with the lixing liquid and its tip is 

 thrust throngh the vitelline membrane, external to the blastoderni 

 but not far from it. The pipette is then pulled 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 

 l)y pressure upon the bulb of the pipette. By pro- 

 perly directing the current it is made to spread out 

 between the vitilline membrane and the blastoderni. 

 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 floats as a loose cake in the tixing 

 tluid. It niust then be removed, at once, to a dish of 

 tixative. This may be done b}^ cutting throngh the 

 vitelline membrane and drawing out tlie blastoderm 

 in a very large pipette or eise by lifting up the 

 blastoderm with a section - lifter. The blastoderm 

 should be tlat and free from yolk. Any supertluous 

 yolk should be washed oft' as the blastoderni lies 

 upside down in the fixative, using a strong but ve)\t/ 

 fhie jet of fixative from a special pipette. 



To obtain good results the Operation must be 

 performed quickly and two possible dithculties guar- 

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

 bles : this is obviated by squeezing out a slow streani 

 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 not rise to the top of the pipette the pipette may be withdrawn, 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 difticulty, the two great hardening and resultiug 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 fixatiou and when it is uecessary to employ them it is 



