Fig. 10. Ovum of Toxopneustes variegatus with the pseudopodia-like projections of the protoplasm penetrating the zona radiata (zr). (After Selenka.)
In Holothuria (Semper) a further differentiation of the germinal cells, not destined to become ova, takes place. They surround the enlarged cell which forms the true ovum, for which they constitute a kind of follicular capsule. This capsule is attached by a stalk to the walls of the ovary, and the ovum lies freely in it except for an area nearly opposite its (the capsule’s) point of attachment, where the ovum adheres to the wall of the capsule. Subsequently the follicle cells which form the capsule fuse together, and form a definite membrane in which only the nuclei remain distinct. Within the membranous capsule there is formed for the ovum an albuminous zona radiata. At the point where the ovum is attached to its capsule this membrane cannot be developed, and therefore remains incomplete. The perforation so formed, becomes the micropyle of the Holothurian egg, which was first discovered by Joh. Müller. The albuminous membrane just described for Holothurians is also found in Asteroids ([fig. 5]) and Echinoids. In these groups there is no proper micropyle, though in Ophiothrix a nutritive passage perforates the membrane at the attachment of the ovum before the period when the ovum becomes free (Ludwig). The formation of the zona radiata has been studied by Selenka. It is secreted by the protoplasm of the ovum, and has a gelatinous consistency, and after it is formed the peripheral layer of the protoplasm of the ovum sends out through it pseudopodia-like processes to absorb nutriment from without. These processes are at first large and irregular, but soon become finer and finer ([fig. 10]), and acquire a regular radiating arrangement. They are withdrawn when the ovum is ripe, but they nevertheless give rise to the finely radiated appearance of the membrane, the radii being in reality delicate pores.
Fig. 11. Transverse section through the pinna of a sexually mature Comatula. (From Gegenbaur, after Ludwig.)
p. Tentacle. g. Lumen of genital rachis. w. Water-vascular vessel. n. Nerve cord. b. Blood-vessel on nerve cord and round genital rachis. cg. Genital canal. cd. Dorsal section of the body cavity. cv. Ventral section of body cavity.
In the Crinoids the generative rachis consists of a tube, the epithelium of which is formed of the primary germinal cells. ([Fig. 11].) While some of these cells enlarge and become ova, the remainder supply the elements for a follicular epithelium, which is established round the ova, exactly as in Holothurians.