ECHINODERMATA. 



developed, and therefore remains in- 

 complete. The perforation so formed, 

 becomes the micropyle of the Holo- 

 thurian egg, which was first discover- 

 ed by Joh. Miiller. The albuminous 

 membrane just described for Holo- 

 thurians is also found in Asteroids 

 (fig. 5) and Echinoids. In these 

 groups there is no proper micropyle, 

 though in Ophiothrix a nutritive pas- FIG. 10. OVUM OF Toxo- 

 sage perforates the membrane at the g-~,Si T 

 attachment of the ovum before the JECTIONS OF THE PROTOPLASM 



e PENETRATING THE ZONA RADI- 



penod when the ovum becomes free ATA ^r). (After Selenka.) 

 (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 arrange- 

 ment. They are withdrawn when the ovum 

 is ripe, but they nevertheless give rise to 

 the finely radiated appearance of the mem- 

 brane, the radii being in reality delicate 

 pores. 



In the Crinoids the generative rachis 

 consists of a tube, the epithelium of which 

 is formed of the primary germinal cells. 

 (Fig. n.) While some of these cells en- 

 large and become ova, the remainder supply 

 the elements for a follicular epithelium, 

 which is established round the ova, exactly 

 as in Holothurians. 



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

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

 cd. Dorsal section of the body cavity, cv. Ventral section of body cavity. 



FIG. 11. TRANSVERSE 

 SECTION THROUGH THE 

 PINNA OF A SEXUALLY 

 MATURE COMATULA. 

 (From Gegenbaur, after 

 Ludwig. ) 



