STRUCTUIIE AND DEVKLOPMENT OF VERTEBRATE OVARY. 383 



however, the last tail gegment of the worm-like '* person," 

 iiaint4y, the radial (figs, i, iv, v, ix, 4) remains fixed, and 

 no plates are added between it and the disc. But a large 

 number are added in succession upon its distal side, so as to 

 build up the arm of the Crinoid with its terminal growing 

 point. 



Consequently, so far as we are entitled to form an opinion 

 based upon the different modes of growth of the arms of the 

 Starfishes and Crinoids respectively, we must adopt one of 

 the two following hypotheses : — Either, 1st, that the in- 

 dividuals composing a Crinoid colony are united together 

 by their tails, and not, like those of an ordinary Starfish 

 colony, by their heads — a view which no one, I suppose, would 

 entertain for a moment ; or, 2nd, that for some reason or 

 other centralisation has proceeded so far, that the principal 

 parts of the bodies of the worm persons which make up a 

 Crinoid have been altogether suppressed, while the tails are 

 developed to a very great extent. The very close cor- 

 respondence, however, in the general anatomy of the disks of 

 the Crinoids and of the Starfishes lend no support to this 

 latter view. Further, if the arms of an OpMura or 

 Brisinga may be considered as representing the body of an 

 Annelid, it is difficult to see why the closely similar arm of 

 a Crinoid should not be regarded in the same light; and 

 although it contains the whole of the genital system, it 

 cannot, according to Haeckel's theory, be anything more 

 than an excessively developed tail. 



On the Structube and Development of the Vertebrate Ovary. 

 By Y. M. Balfour, M.A., FeRow' of Trinity College, Cam- 

 bridge. (With Plates XVII, XVIII, XIX.) 



Contents. 



Introduction. 

 Development of Elasmobranch Ovary, p. 384 — 418. 

 Literature, p. .384. Ovarian Eidge in Scyllium, p. 385. Formation of Per- 

 manent Ova and of the Follicular Epithelium, p. 392. 



Post-embryonic Development of the Ova, p. 401 — 415. 

 General Observations, Methods, &c., p. 401. Egg: Membranes, p. 401. 

 Follicular Epithelium, p. 403. The Vitellus and the Development ofilie 

 Yolk-spherules, p. 408. Germinal Vesicle, p. 412. Fonnaiion of 

 fresh Ova and Ovarian Nests in the post-embryonic periods, p. 413. 

 VOL, XVII I. NEW SER. C C 



