2CA PEACTICAL DIRECTIONS. [APP- 



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



With the im incubated 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. Stnface view transparent from above. 

 Observe the absence of the central opacity. 



E. Surface view transparent from underneath. 

 Nothing further to be observed than from above. 



F. As an opaqne 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 epiblast. 



b. The lower layer cells not as }^et differentiated 

 into mesoblast and hypoblast. 



c. The thickened ed^e of the blastoderm. 



d. The segmentation cavity and formative cells. 



VIII. Examination of the 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 esf^s at the various stages of sea:- 

 mentation, may usually be obtained without a great 

 unnecessary sacrifice of hens. For making sections, 

 the yolk must in all cases be hardened as a whole, 

 which may be done as recommended in VI. G. 



