262 PRACTICAL DIRECTIONS. [APR 



4. The notochord as an opaque streak along the floor 

 of the medullary groove. 



E. Surface view transparent, from below. 



Same points to be seen as from above, but less 

 clearly. 



F. Embryo as an opaque object. 



As an opaque object, whether the embryo is hard- 

 ened in situ or after being removed from the yolk, 

 the same points are to be seen as when it is viewed 

 as a transparent object, with the exception of the 

 notochord and protovertebrse (vide D). The various 

 grooves and folds are however seen with far greater 

 clearness. 



G. Sections. 



Two methods of hardening may be employed; 

 (1) with the embryo in situ, (2) after it has been 

 removed. 



To harden the blastoderm in situ the yolk must 

 be hardened as a whole. After opening the egg either 

 leave the yolk in the egg-shell or pour it out into a 

 Berlin capsule; in any case freeing it as much as 

 possible from the white, and taking especial care to 

 remove the more adherent layer of white which im- 

 mediately surrounds the yolk. 



Place it in a weak solution of chromic acid (first 

 of 1 p.c. and then of *5 p.c.) with the blastoderm upper- 

 most and leave it in that position for two or three days. 



Care must be taken that the yolk does not roll 

 about ; the blastoderm must not be allowed to alter 

 its position: otherwise it may be hard to find it when 

 everything has become opaque. If at the end of the 

 second day the blastoderm is not sufficiently hard, 

 the strength of the solution should be increased and 

 the specimen left in it for another day. 



After it has become hardened by the chromic acid, 

 the yolk should be washed with water and treated 

 successively with weak and strong spirit, vide II. G. 

 After it has been in the strong spirit (90 p. c.) for two 

 days, the vitelline membrane may be safely peeled off 



