528 
PROFESSOR W. THOMSON ON THE EMBRTOGENY OF ANTEDON 
with in the pseudembryos and young of some of the other Echinoderm orders. At this 
early period no general body-cavity can be detected separating the wall of the stomach 
from the body. The stomach seems to be simply excavated in the structureless body- 
substance, and the organism corresponds generally with the Ccelenterate type. The 
external sarcode-layer still retains much the same character which it possessed in the 
pseudembryonic stage. Its basis is transparent and structureless, with imbedded pyri- 
form oil-cells, endoplasts, and granules. 
The stem now gradually lengthens, by additions to either end of the sheaf-like calca- 
reous cylinders which form the axes of the stem joints, and by the addition of new rings 
which rapidly become filled up by the vertical tissue, at the top of the stem, imme- 
diately beneath the rudiment of the centro-dorsal plate (Plate XXVI. fig. 2). The disk 
of attachment becomes opaque by the addition of calcareous matter, and is firmly fixed. 
The centro-dorsal ring (Plate XXVI. fig. 2) is more definite in form, though it is still 
simply perforated in the centre, and in connexion with the sarcode-axis of the stem, 
and bears no traces of dorsal cirri. The basals expand and form a wide, nearly con- 
tinuous cup. By the rapid expansion of the body, five diamond-shaped spaces are left 
at the points where the upturned angles of two oral plates are opposed to the bevelled- 
off upper angles of two adjacent basals. In these spaces cylindrical spicula appear, 
which soon become club-shaped, dichotomize, branch, and anastomose into delicate 
net-like superficial plates, irregularly oval, slightly produced superiorly, their upper, 
narrower portions resting beneath, and supporting, the gradually extending sarcode pro- 
jections which are terminated by the azygous tentacles (Plate XXVII. fig. 1). The 
equatorial portion of the body, the band between the upper edges of the basals and the 
lower edges of the orals, now rapidly expands. The five young arms extend outwards, 
their bases carrying out with them a zone of sarcode which gives the central portion of the 
body a great additional width. The oral plates maintain their original position, so that 
they are now completely separated from the basals by this intervening equatorial band ; 
and are left, a circle of five separate plates, each enclosed in its sarcode-lobe, on the 
centre of the upper surface surrounding the mouth, and enclosing the ten non-extensile 
tentacles only. The first radial plates begin to thicken, especially towards the upper 
margin, and this thickening is produced by the growth, beneath the cribriform super- 
ficial calcareous film, of a longitudinal mass of tissue of the same character as that 
which forms the cylindrical axis of the stem joints. On the lower surface of each arm, 
in linear series, immediately above the first radials, two spicula, horseshoe-shaped, with 
the opening above, appear almost simultaneously, and become quickly filled up with 
elongating sheaves of longitudinal trellis-work. These extend along beneath the 
extending arms, and indicate the second radials and the radial axillaries. 
The upper surface of the arms now becomes grooved by the development, on either 
side of the central vessel, of a series of delicate crescentic leaves. These leaves are 
hollow, communicating by special apertures with the radial vessel, and filled with fluid 
from it. At the base of each of the leaves there is a pair of tentacles forming a group 
