MUSEUM OF COMFAItATIVE ZOOLOGY. 9 



formed terminal. It would seem to indicate that the madreporic open- 

 ing was a fixed point of departure, as far as the age of plates is con- 

 cerned. Unfortunately for this conclusion, it is only conjectural that 

 such is the case, and the terminals seem to arise simultaneously. On 

 the other hand, my observations on Asterias do not agi-ee with Lud- 

 wig's on Asterina, that the madrepoi'ic plate, or the genital near the 

 madreporic opening, is larger than the others, or has a predominance in 

 size in early stages.* 



On each side of the radial water tubes, near the circular tube, in their 

 early condition, are formed tlie ambulacral oral plates, am, which are 

 placed at first in parallel pairs and are ten in number. The pro- 

 gi-ess of the growth of these plates and the addition of new ambu- 

 lacral rafters will be treated of in a special account of the gTowth of 

 these plates. It is to be noticed that the oral plates are in parallel 

 pairs at first, like the spoon-shaped plates, and other ambulacrals of 

 Amphiura. 



Prominent among the calcareous formations which mark this stage 

 (PI. I. fig 3) in the development of the starfish are the abactinal 

 spines, s/9, which appear first on the terminals, and later on the genitals. 

 Their early appearance introduces a morphological question in the dis- 

 cussion of the homologies of the Echinoderms. 



In a disk-shaped starfish (PL II. fig. 1) which follows hard on the 

 last, the brachiolaria has been almost wholly absorbed, and the brachio- 

 larian notch is very much diminished in width. The relationship of 

 the madreporic tube and the madreporic opening to the genital plate, 

 g^, is indicated by an arrangement of the branches of the calcareous 

 spicules on the outer border. There is no true madreporic body or 

 superficial calcification until after the stone canal has begun to form, 



* In this way I interpret what Ludwig says (pp. 49, 50), in speakinsr of the 

 eleven first-formed abactinal plates : " 1st doch eines, welclies den iibrigen zehn 

 fast iniraer etwas voraus ist in Bezug auf die Zeit seines ersten Auftretens. . . . Aus 

 diesem Skelettstiick wird die Madreporenplatte des Seesternes." 



In one or two specimens the predominance in size of the interradial [pace. Car- 

 penter) plate near the madreporic opening was thought to exist, but in others no 

 such prominence was noticed. No preparation which was made shows the " mad- 

 reporic plate " without one or two other genitals, and I have none with this plate 

 without terminals. 



As Ludwig has shown in Asterina, all the terminals and other genitals do not 

 appear always simultaneously in Asterias. The dorsocentral is generally' belated, 

 and one or two genitals may be developed after the others. The same may also 

 be true of the terminals. (Ludwig, Entwicklungs-geschichte ilev Asterina ^ibbosa, 

 Forbes, Zeit. f. Wiss. Zool , Vol. XXXVII., 1882.) 



