454 PEACTICAL DIRECTIONS. [APP. 



derm in situ at this age. The student will however 

 not fail to notice the halones, which can be seen 

 forming concentric rings round the blastoderm. 



C. Removal of the embryo. 



Two methods of hardening can be adopted at 

 this age. One of these involves the removal of the 

 blastoderm from the yolk, as in II. 0. In the other 

 case, the yolk is hardened as a whole. If the latter 

 method be employed, the embryo cannot be viewed 

 as a transparent object. 



In the cases where the blastoderm is removed 

 from the yolk, the manipulation is similar to that 

 described under II. 0, with the exception of more 

 care being required in freeing th« blastoderm from 

 the vitelline membrane. 



D. Surface view transparent, from above. 

 Observe : — 



1. The medullary groove between the two medullary 

 folds, whose hind ends diverge to enclose between 



them the end of the primitive groove. 



2. The headfold at the end of the medullary groove. 



3. The one or two pairs of mesoblastic somites flanking 

 the medullary groove. 



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. 



