THE OVUM. 



17 



referred to the special part of this chapter. Suffice it to say that 

 none of the known modes of nutrition indicate that the ovum becomes 

 a compound body any more than the fact of an Amoeba feeding on 

 another Amoeba would imply that the first Amoeba ceased thereby to 

 be a unicellular organism. 



The constitution of the ovum may be considered under three 

 heads:- - 



(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 important 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 nucleus, which is very possibly connected 



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



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

 gv. Germinal vesicle. 



B. SINGLE PSEUDOCELL OF HYDRA. 



B. E. * 



