In Hydra the locality where the ova are developed only becomes specialized at the time when an ovum is about to be formed. At one or more points the interstitial cells of the epiblast increase in number and form a protuberance of germinal cells, which may be called the ovary. In this ovary a single ovum is formed by the special growth of one cell. (Kleinenberg, No. [9].) In the free and attached gonophores of Hydrozoa, the ova appear either around the walls of the stomach, or the radial canals, or around other parts of the gastro-vascular canals.
Fig. 8. Ripe Ovum Of Epibulia aurantiaca. The Germinal Vesicle has become invisible without reagents.
Copied from Metschnikoff, “Entwicklung der Siphonophoren.” Zeitschrift f. wiss. Zool., Vol. XXIV. 1874.
p.d. Peripheral layer of denser protoplasm. p.m. Central area consisting of a protoplasmic meshwork.
Their close relations to the gastrovascular canals are probably determined by the greater nutritive facilities thereby afforded. (Hertwig, No. [8].)
In the permanent Medusa forms the ova have similar relations to the gastro-vascular system. Amongst the Actinozoa the ova are usually developed between the epiblast and the hypoblast in the walls of the gastric mesenteries. Amongst the Ctenophora the ova are situated in close relation with the peripheral canals of the gastro-vascular system, which run along the bases of the ciliated bands. There are many examples amongst the Cœlenterata of ova which retain in their mature state the very simple constitution which has been described as characteristic of all young ova; and which are, when laid, absolutely without any trace of a vitelline membrane or chorion. In many other cases both amongst the Medusæ, the Siphonophora, and the Ctenophora, the ripe egg exhibits a distinction into two parts. The outer part is composed of a dense protoplasm, while the interior is composed of a network or more properly a spongework of protoplasm enclosing in its meshes a more fluid substance. ([Fig. 8].)
In some cases the ovum while still retaining the constitution last described becomes invested by a very delicate membrane. Such is the constitution of the ripe ovum of Hippopodius gleba amongst the Siphonophora[16] and of the eggs of Geryonia amongst the permanent Medusæ[17]. The ripe eggs of the Ctenophora usually present a similar structure[18]. After being laid they are found to be invested by a delicate membrane separated by a space filled with fluid from the body of the ovum. The latter is composed of two layers, an outer one of finely granular protoplasm and an inner layer consisting of a protoplasmic spongework containing in its meshes irregular spheres. These latter are stated by Agassiz to be of a fatty nature, and it is probable that in most cases where a protoplasmic network is present, this alone constitutes the active protoplasm and that the substance which fills up its meshes is to be looked on as a form of food-yolk or deutoplasm, though it appears sometimes to have the power of assimilating the firmer yolk particles.
The membrane which invests the ovum of many of the Cœlenterata is probably a vitelline membrane.