464 PRACTICAL DIRECTIONS. [APP. 



D. The fully segmented ovum. 70 hours old. 



The fully segmented ovum is found in the uterus 

 at its anterior end close to the place where the 

 ovidiict opens into the uterus. 



To obtain this stage the uterus must be slit open 

 and examined carefully with a dissecting lens : the 

 ovum will be seen as a somewhat opaque spot on the 

 glistening moist mucous epithelium of the uterus. 



It may be treated in the manner described under 

 B. a., but the segments being closely pressed to- 

 gether their outlines are not rendered distinct by 

 this method. A more advantageous mode of treatment 

 is the following : wash the ovum rapidly in distilled 

 water, and place it in a 1 p.c. solution of silver 

 nitrate for about 3 minutes : then expose it to the 

 light in a dish of distilled water until it be tinged 

 a brown colour. 



The brown colour is due to the reduction of the 

 silver, which takes place chiefly in the cement sub- 

 stance between the cells and thus defines very exactly 

 their size and shape. The ovum may now be treated 

 with glycerine and mounted as described in B. 



The points to be observed are : — 



1. The division of the segmentation spheres into the 

 layers — an outer layer of cubical hyaline cells, and 

 an inner of rounded granular cells. 



2. The blastopore of van Beneden. 



3. The presence of a thin layer of mucous outside 

 the concentrically ringed albuminous coat of the 

 ovum. 



