MUSEUM OF COMPARATIVE ZOOLOGY. #21 
siz,* Metschnikoff,f and Ludwig. The dorsocentral early takes its per- 
manent place, and remains near the centre of the abactinal region of the | 
disk ; the radialia are subsequently to formation pushed nearer the peri- 
phery by the interposition of new plates between them and the dorsocen- 
tral. They are never, however, in Amphiura pressed to the very periphery 
of the body, and consequently never extend out on the arms. In early 
stages the dorsocentral § is not formed, the five radialia originating first. 
The dorsocentral in the young Amphiura (Fig. 13) occupies the centre 
of the abactinal surface of the body, and is irregularly pentagonal. Its 
origin was not traced, but there is no reason to question the observations 
of Ludwig and Schultze|| that the radials appear before the dorsocentral. 
In the youngest pentagonal embryo which was studied (Fig. 12) in which 
the umbilicus (w) was still unbroken, the five radialia (rp) were well 
formed and of comparatively large size, while the. dorsocentral (dc) was 
small, indicating apparently a later formation. Ludwigt says, “Ganz 
unterdriick aber wird die Entwickelung eines Centrale niemals.” The 
radialia or primary radials are among the first plates to form, and origi- 
nate while yet the Amphiura is in a bilaterally symmetrical condition. 
It is extremely difficult to distinguish them in earliest conditions from 
the network at the anal end of the provisional pluteus rods (Fig. 8), but 
they are among the first plates of the Amphiura to form. The question 
of whether they antedate in formation any of the plates of the actinal 
region of the disk will probably be correctly answered in the affirmative. 
They probably appear before the first pair of adambulacral (gq. v.) plates 
and undoubtedly before the terminalia (gq. v.). 
A. Agassiz states:{] “‘ We have the most positive proof of the origin of 
the dorsocentral plate of starfishes and Ophiurans as a single Y-shaped rod 
appearing simultaneously with the five basals.” Ludwig, on the other 
hand, states : ‘‘ Diese sechste Platte, das Centrale tritt bei Amphiura squa- 
mata in der Regel spater auf als die fiinf Radialia.” Ludwig holds ** 
* Embryology of Echinoderms, Mem. Amer. Acad, Arts and Sci., III. 1856. 
+ Op. cit., p. 18; Pl. IV. Fig. 17. 
t Op. cit.,p. 195. 
§ The reader is referred in this connection to Fig. 16, Pl. VII., in my paperon the 
development of Echinarachnius (Bull. Mus. Comp. Zodl., X11. 4). In this figure, 
which is of a young sea-urchin, the calcareous network of the dorsocentral plate (c p/) 
has not begun to form, although that of the ring of plates surrounding it is well 
marked. 
ll Op. cié., PL I. Fig. 4. 
‘| Report on the Echini, Mem. Mus. Comp. Zoél., Vol. X. No. 1. 
** Pace 187. 
