THE OVUM. 



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(1) The body of the ovum. 



(2) The nucleus or germinal vesicle. 



(3) The investing membranes. 



The body of the ovum. The essential constituent of the 

 body of the ovum is an active living protoplasm. As a rule 

 there are present certain extraneous matters in addition, which 

 have not the vital properties of protoplasm. The most impor- 

 tant of these is known as food-yolk, which appears to be 

 generally composed of an albuminoid matter. 



The body of the ovum is at first very small compared with 

 the germinal vesicle, but continually increases as the ovum 

 approaches towards maturity. It is at first comparatively free 

 from food-yolk ; but, except in the rare instances where it is 

 almost absent, food-yolk becomes deposited in the form of 

 granules, or highly refracting spheres, by the inherent activity 

 of the protoplasm during the later stages in the ripening of 

 the ovum. In many instances the protoplasm of the ovum 

 assumes a sponge-like or reticulate arrangement, a fluid yolk 

 substance being placed in the meshes of the reticulum. The 

 character of the food-yolk varies greatly. Many of its chief 

 modifications are described below. There is not unfrequently 

 present in the vitellus a peculiar body known as the yolk 



FIG. 4. A. OVUM OF HYDRA IN THE AMCEBOID STATE, WITH YOLK SPHERULES 

 (PSEUDOCELLS) AND CHLOROPHYLL GRANULES. (After Kleinenberg. ) 



gi>. Germinal vesicle. 



B. SINGLE PSEUDOCELL OF HYDRA. 



