2G4 PEACTICAL DIRECTIONS. [APP. 



C. Removal of the blastoderm. Vide VI. C. 



With the unincubated blastoderm still greater care 

 is required in removal than with the 20 hours' blasto- 

 derm, and there is no special advantage in doing so 

 unless it is intended to harden it with osmic acid. 



D. Surface view transparent from above. 

 Observe the absence of the central opacity. 



E. Siuface view transparent from nnderneath. 

 Nothing further to be observed than from above. 



F. As an opaque object. 

 There is nothing to be learnt from this. 



G. Sections. 



Manipulation as in VI. G. 



Only one section is required, viz. one taken through 

 the centre of the blastoderm, shewing: 



a. The distinct ejyiblast. 



h. The low6r layer cells not as yet differentiated ; 

 into mesoblast and hypoblast. 



c. The thickened edge of the blastoderm. 



d. The segmentation cavity and /urinative cells. 



VIII. Examination of tlie process of Segmentation. 



To observe the process of segmentation it will be 

 found necessary to kill a number of hens which are 

 laying regularly. The best hens lay once every 24 

 hours, and by observing the time they usually lay (and 

 they generally lay pretty regularly about the same 

 time), a fair guess may be made beforehand as to 

 the time the egg has been in the oviduct. By this 

 means a series of esrsfs at the various stashes of sejr- 

 mentation, may usually be obtained without a great 

 unnecessary sacrifice of hens. For making sections, 

 the yolk miist in all cases be hai'dened as a whole, 

 which may be done as recommended in. VI. G. 



