IPP.] BLASTODERM OF TWENTY HO UBS. 261 



In the hardened specimens, especial attention 

 should be paid to the changes which take place in 

 the parts forming the boundaries of the mouth. 



In making sections, it will probably be found 

 impossible to stain the embryo as a whole; in 

 this case, the individual sections must be stained 

 separately. 



VI. Examination of a Blastoderm of 20 hours. 



A. Opening the egg, as in II. A. 



B. Examination in situ. 



It will not be found possible to make out anything 

 very satisfactory from the examination of a blasto- 

 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. C. In the other 

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

 of these is the most satisfactory method for sections ; 

 but if 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. C, with the exception of more 

 care being required in freeing the 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 head-fold at the end of the medullary groove. 



3. The one or two pairs of protovertebrce flanking 

 the medullary groove. 



