32 Ech. 
ECHINODERMATA. 
process of degeneration of their elements, either at their complete 
development, or in various phases of their evolution. The germinal 
vesicle and spot are first attacked by a hyaline or colloid degeneration, 
rarely by a fatty or chromatolytic degeneration. The vitellus and vitel- 
line membrane are next attacked, the former being broken up by swelling 
of the lecithin. In correspondence with this destruction of the ovarian 
parenchyma, there is a constant renovation of the same, the elements for 
which are furnished by the cells of the egg-follicle. In some eggs the 
germinal vesicle acquires a characteristic form, in which the nuclein is 
pushed out and expelled. This process appears to be for the purpose of 
ridding the ovum of the most important element when it is not destined 
to be fertilized. Thus the ovary shows a continual biological activity in 
the generation and destruction of its elements. 
Russo (4) describes the spermatogenesis and oogenesis of various 
Ophiuroida , and the morphology of the genital apparatus. The sperma- 
tozoa are produced from a succession of elements (spermatogonium, 
spermatocyte, spermatid), produced from the germinal epithelium. The 
elements arrange themselves in distinct layers. The mother-cells of the 
ova arise from the basal epithelium, and multiply in the ovarial tubes 
by karyokinesis. Some become primitive ova, others follicular cells. 
The germinal vesicle has at first a reticulum, in which accumulations of 
chromatin take place at the nodes. The germinal spot at first is pro- 
duced by an accumulation of chromatin, but finally becomes an isolated 
body of special structure. The vitellus forms two zones, one close to the 
germinal vesicle, with a fibrillar structure, and intensely coloured ; the 
other reticular and almost colourless. The follicle, when mature or 
nearly so, constitutes a syncytium. The ova constantly undergo destruc- 
tion and renovation [</. Russo (1)]. The ova and spermatozoa in 
Ophiuroida have a common origin. Genital orifices always exist. A 
comparative account is given of the Gonads of various Ophiuroida. 
For the minute structure of the ovum of Strongylocentrotus lividus and 
Sphcerechinus brerispinoaus , see Schneider. 
Wachsmuth & Springer divide the skeletal elements of a Crinoid 
into primary and secondary. The primary elements include (1) abactinal 
plates, viz., stem ossicles, infrabasals, basals, radials, and all brachials, 
(2) actinal plates, viz., orals and all plates of the ambulacra. The 
secondary or supplementary elements are all the interradial, inter- 
brachial, and interambulacral plates, including the anal plates and plates 
of the tube, or sac. 
The simplest form of tegmen in Crinoids consists of five interradial 
plates meeting over the mouth, the orals. To increase the calycal cavity, 
interradial plates were developed between the radials, while interambu- 
lacral plates, as well as extensions of the ambulacral plates, gradually 
intervened between orals and radials. The orals might disappear or 
remain. As the lower parts of the arms became incorporated in the 
dorsal cap, more and more ambulacral and interambulacral plates came 
into the tegmen. The plates of the tegmen were at first small and 
