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 with the formation of the food-yolk. It is found in many Arachnida, Myriapoda, Amphibia, etc.[13] [13] For details on the yolk nucleus _vide_ Balbiani, _Leçons s. l. Génération d. Vertébrés_. Paris, 1879. In this work the author maintains very peculiar views on the nature and function of the yolk nucleus, which do not appear to me well founded. [FIG. 4. _A._ OVUM OF HYDRA IN THE AMOEBOID STATE, WITH YOLK-SPHERULES (PSEUDOCELLS) AND CHLOROPHYLL GRANULES. (After Kleinenberg.) _gv._ Germinal vesicle. _B._ SINGLE PSEUDOCELL OF HYDRA.] More important for the subsequent development than the variation in the character of the food-yolk is its amount and distribution. In a large number of forms it is distributed unsymmetrically, the yolk being especially concentrated at one pole of the ovum, the germinal vesicle, surrounded by a special layer of protoplasm comparatively free from food-yolk, being placed at the opposite pole. In the Arthropoda it has in most instances a symmetrical distribution. Further details on this subject are given in connection with the segmentation; the character of which is greatly influenced by the distribution of food-yolk. The body of the ovum is usually spherical, but during a period in its development it not unfrequently exhibits a very irregular amoeboid form, _e.g._ Hydra (fig. 4), Halisarca. [FIG. 5. UNRIPE OVUM OF TOXOPNEUSTES LIVIDUS. (Copied from Hertwig.)]