THE OVUM. 



FIG. 10. OVUM OF Toxo- 

 PNEUSTES VARIEGATUS WITH THE 

 PSEUDOPODIA-L1KE PBOJECTIONS 

 OF THE PROTOPLASM PENETRATING 



THE ZONA RADIATA (ZJ - ). (After 



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 incom- 

 plete. The perforation so formed, becomes 

 the micropyle of the Holothurian egg, 

 which was first discovered by Joh. Miiller. 

 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 CLudwig). 

 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 with- 

 out. These processes are at first large and 

 irregular, but soon become finer and finer (fig. 

 10), and acquire a regular radiating arrange- 

 ment. They are withdrawn when the ovum 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. 



In the Crinoids the generative rachis con- 

 sists of a tube, the epithelium of which is 

 formed of the primary germinal cells. (Fig. 

 11.) While some of these cells enlarge and be- 

 come ova, the remainder supply the elements 

 for a follicular epithelium, which is established 



round the ova, exactly as in Holothurians. 



J wig.) 



p. Tentacle, g. Lumen of genital rachis. iv. Water-vascular vessel, n. Nerve 

 cord. b. Blood-vessel on nerve cord and round genital rachis. c g. Genital canal. 

 c. d. Dorsal section of body cavity, c v. Ventral section of body cavity. 



c>7 



FIG. 11. TRANSVERSE. 

 SECTION THROUGH THE 

 PINNA OF A SEXUALLY 



MATURE COMATULA. (From 



Gegenbaur, after Lud- 



