APP.] TWENTY HOUKS EMBRYO. 455 



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 mesoblastic somites (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 picric acid or a weak solution of chromic 

 acid (first of '1 p.c. and then of '5 p.c.) with the 

 blastoderm uppermost 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, if chromic acid be used, 

 should be increased and the specimen left in it for 

 another day. 



After it has become hardened by the acid, the 

 yolk should be washed with water and treated sue- 



