252 PRACTICAL DIRECTIONS. [APP. 



canal, protovertebraB and commencing cleavage of 

 the mesoblast. 



6. Through a point where the medullary canal is still 

 open, shewing the mode in which its closing takes 

 place. 



A good series of transverse sections having been 

 obtained, longitudinal vertical ones may be made 

 in a similar manner. These are however much 

 more difficult to manage ; and are moreover chiefly 

 of use to compare with the transverse ones. 



GG. Preservation of the embryo as a whole. 



Embryos of this or an earlier day may be easily 

 preserved whole as microscopic objects; but the value 

 of such preparations is very slight, and they are per- 

 haps not worth the trouble. The best method is 

 probably to place the embryos in the picric acid 

 solution for a short time, and then successively in 

 weak, strong and absolute alcohol and finally to mount 

 them, in glycerine or, after staining and treatment 

 with oil of cloves, in balsam. For shewing some 

 points, treatment with osmic acid for a short time and 

 then with alcohol and finally with glycerine may be 

 adopted. 



Whole embryos of a later date cannot be satis- 

 factorily preserved as microscopic objects. 



III. Examination of an Embryo of about 48—50 hours. 



A. Opening the egg — as in II. A. 



B. Examination of the blastoderm in situ. 



Observe 



1. The form of the embryo, which is much more 

 distinct than at the earlier sta^e. 



2. The beating of the heart. 



3. The general features of the circulation. 



