60 ATLAS OF BIOLOGY. 



FIG. VII. The dart-sac in transverse section, x 2. 



Careful examination shows that the blades of the dart are slightly twisted, so that it 

 must leave its sac in the fashion of a revolving gun-shot. 



FIG. VIII. The spermatozoa. F. 3. 



i. Two isolated spermatozoa. 



ii. A spermatozoan aggregate. 



iii. A group of immature spermatozooids. 



FIG. IX. Two ova, obtained, as were the above spermatozoa, by teasing up a small 

 piece of the ovotestis in eosin solution. D. 3. 



FIG. X. The ovum of the Pond snail* (Lymnaus stagnalis) during segmentation, seen 

 from beneath. D. 2. 



FIG. XI. Side-view of the same. 



FIG. XII. The same, at the gastrula stage, in optical section. D. 2. 



FIG. XIII. The early larva of the same. Surface view from the side. D. 2. 



FIG. XIV. The above in optical section. 



FIG. XV. A twelve days' embryo of the same, from the left side. D. 2. 



All from life, after treatment with 1 p.c. osmic acid. Figs. X. to XIII. after Lankester 

 (72). The cilia are exaggerated. 



* This type is chosen on account of the facility with which its larvte can be obtained. 



