APP.] EXAMINATION OF OVUM. 463 



very gradually more and more dense. It should 

 be mounted in glycerine in which 1 p.c. formic 

 acid has been mixed to prevent fungoid growths. 

 Care must be taken that there is no pressure 

 upon the ovum this being insured by the inser- 

 tion of a couple of slips of paper one on each side 

 of the ovum under the cover glass. 

 b. Another method of preservation is used, but 

 does not appear to us so successful as the one 

 already described. It consists of an immersion 

 of the ovum for 5 minutes in i to J p.c. osmic 

 acid, subsequent treatment with Mtiller's fluid 

 for two or three days, and finally mounting in 

 glycerine. 



C. Examination of the ovum. 



The most instructive stages to observe are ova of 



a. 18 hours old, when four segmentation spheres 

 will be observed. 



b. 36 hours old when the segmentation is more 

 advanced and the spheres numerous. 



The chief points to be noted are : 



1. The number and size of the segmentation spheres; 

 in each of which, when treated as described in B. a., 

 a large deeply stained nucleus will be visible. The 

 spheres themselves are also stained slightly. 



2. The presence of one or two polar bodies on the 

 outer side of the segments in ova of not more than 

 48 hours old: these also are slightly stained. 



3. The zona radiata immediately surrounding the 

 segments, and 



4. The thick albuminous coat, marked with con- 

 centric rings. 



