APP.] HAKDENING THE WHOLE YOLK. 263 



and the blastoderm and embryo will be found in 

 situ. The portion of the yolk containing them must 

 then be sliced off with a sharp razor, and placed in 

 absolute alcohol. 



The staining, &c. may be effected in the ordinary 

 way. 



If osmic acid, which we believe will be found 

 especially serviceable for these early stages, is em- 

 ployed, it will be necessary to remove the blastoderm 

 from the yolk before treating it with the reagent. 



The following transverse sections are the most im- 

 portant at this stage : 



1. Through the medullary groove, shewing 



a. The medullary folds with the thickened meso- 

 blast. 



b. The notochord under the medullary groove. 



c. The commencing cleavage of the mesoblast. 



2. A section through the region where the medullary 

 folds diverge, to enclose the end of the primitive 

 groove, shewing the greatly increased width of the 

 medullary groove, but otherwise no real alteration 

 in the arrangement of the parts. 



3. A section through the front end of the primitive 

 groove with the so-called axis cord underneath it, 

 while on each side of it are still to be seen the 

 medullary folds. 



4. A section through the primitive groove behind this 

 point, shewing the typical characters of the primitive 

 groove. 



VII. Examination of an unincubated Blastoderm. 



A. Opening the egg. Vide II. A. 



B. Examination of the blastoderm in situ. 



Observe the central white spot and the peripheral 

 more transparent portion of the blastoderm and the 

 halones around it. 



