28 ECHINODERMATA. 



other cases they retain their independence and serve for the late 

 nutrition of the embryo. In both instances they retain the faculty, 

 normally possessed by ova, of forming yolk particles in their proto- 

 plasm. 



ECHINODERMATA. 



fm) C. K. Hoffmann. " Zur Anatomic d. Echiniden u. Spatangen." Nieder- 

 landisch. Archiv f. Zoologie, Vol. i. 1871. 



(16) C. K. Hoffmann. "Zur Auatomie d. Asteriden." Niederlandisch. Archiv 

 f. Zoologle, Vol. ir. 1873. 



(17) H. Ludwig. "Beitrage zur Anat. d. Criuoiden." Zeit.f. wi.is. Zool., Vol. 

 xxvm. 1877. 



(18) Joh. Muller. " Ueber d. Canal in d. Eiern d. Holotlaurien." Uliiller't 

 Archiv, 1854. 



(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. 11, 6). 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 Coslenterata. 



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 under- 

 go 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 en- 

 larged 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 



