266 PRACTICAL DIRECTIONS. [APP. 



The blastoderm when under examination must be 

 protected by a cover-slip with the usual precautions 

 against pressure and evaporation, and a hot stage 

 must also be employed. 



Fresh objects so prepared require to be examined 

 with a considerable magnifying power (400 to 800 

 diameters). From a series of specimens between 30 

 and 40 hours old all the points we have mentioned in 

 Chapter IV. 6 can without much difficulty be observed., 



Especial attention should be paid in the earlier 

 specimens to the masses of nuclei enveloped in proto- 

 plasm and connected with each other by protoplasmic 

 processes ; and in the later stages to the conversion of 

 these nuclei into blood corpuscles and of the proto- 

 plasmic processes into capillaries, with cellular walls. 



Blastoderms treated in the following ways may be 

 used to corroborate the observations made on the 

 fresh ones. 



1. With gold chloride. 



Immerse the blastoderm in gold chloride (*5 p. c.) 

 for one minute and then wash with distilled water 

 and mount in glycerine and examine. 



By this method of preparation, the nuclei and 

 protoplasmic processes are rendered more distinct, 

 without the whole being rendered too opaque for 

 observation. 



The blastoderm after the application of the gold 

 chloride should become a pale straw colour ; if it 

 becomes in the least purple, the reagent has been 

 applied for too long a time. 



2. With potassium bichromate. 



Immerse in a 1 p. c. .solution for one day and then 

 mount in glycerine. 



3. With osmic acid. 



Immerse in a "5 p. c. solution for half an hour and 

 then in absolute alcohol for a day, and finally 

 mount in glycerine. 



