456 PRACTICAL DIRECTIONS. [APP. 



cessively with weak and strong spirit, vide 1. B. 

 After it has been in the sti-ong spirit (90 p.c.) for two 

 days, the vitelline membrane may be safely peeled ofi" 

 and the blastodei-m and embryo will be found in 

 situ. The portion of the yolk containing them must 

 then be sliced off with a sharp razoi', and placed in 

 absolute alcohol. 



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

 way. 



If osmic acid, which we believe will be found 

 serviceable for these early stages, is employed, it will 

 be necessary to remove the blastoderm from the yolk 

 before treating it witli 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 mesohlast. 



2. Through the region where the medullary folds 

 diverge, to enclose the end of the primitive groove, 

 shewing the greatly increased width of the medul- 

 lary groove, but otherwise no real alteration in 

 the arrangement of the parts. 



3. 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 medul- 

 lary folds. 



4. Through the primitive groove behind this point, 

 shewing the typical characters of the primitive 

 groove. 



