THE OVUM. 35 



(19) C. Semper. Holothurien. Leipzig, 1868. 



(20) E. Selenka. Befruchtung d. Eies v. Toxopneustes variegatus, 1878. 



[ Vide also Ludwig (No. 4), etc.] 



The eggs of the Echinodermata present in their development 

 certain points of interest. 



The ovaries themselves are usually surrounded by a special 

 vascular dilatation. In the Asteroidea, the Echinoidea, and the 

 Holothuroidea the organs have the form of sacks ; specially 

 surrounded in the two former groups, and probably the latter, 

 by a vascular sinus formed as a dilatation of one of the 

 generative vessels. In the Crinoids they have the form of a 

 hollow rachis completely surrounded by a blood-vessel. (Fig. 

 n, &) The proximity of the ovaries (generative organs) to the 

 vascular system in these forms has clearly the same physiological 

 significance as the proximity of the ovaries (generative organs) 

 to the radial vessels in the Ccelenterata. 



In the Asteroidea, the Echinoidea and the Holothuroidea the 

 ovaries have the form of sacks lined by an epithelium of germinal 

 cells, and the ova are formed by the enlargement of these cells, 

 which, when they have reached a certain size, become detached 

 from the walls, and fall into the cavity of the ovarian sack. In 

 Toxopneustes (Selenka) and very probably in other forms only 

 a few of the epithelial cells undergo conversion into ova : the 

 remainder undergo repeated division, and, as in so many other 

 " cases, are eventually employed in the nutrition of the true ova. 

 In the nearly ripe ova of Asterias Fol has described a flattened 

 follicular epithelium the origin of which is unknown. 



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 cap- 

 sule. 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 



32 



