538 
PROFESSOR W. THOMSON ON THE EMBRYO OENY OF ANTEDON 
cations in form, in the mature Antedon. These ten plates appear simultaneously as 
delicate spicula imbedded within the firm peripheric layer of the pseudembryo, usually 
only a few hours after its escape from the vitelline sac, and before there is any trace of 
the permanent organs of the embryo. 
The spicula are hollow throughout. They are at first simple and cylindrical ; shortly 
they become club-shaped at each end; each thickened end then divides into two 
diverging branches, equal in length to the original rod ; these fork in their turn, till on 
their second bifurcation their branches meet and coalesce with the corresponding 
branches from the opposite end of the original spiculum. By thus constantly branching 
and anastomosing on one plane, the spiculum extends into a delicate net-like plate, the 
meshes of which are at first irregularly hexagonal, but afterwards become rounded. 
The extending calcareous tubes are constantly closed, and constantly hollow to the end. 
They appear to grow by the molecular removal of calcareous matter from the back of 
the growing point, and its deposition in advance. At first all the ten plates are round ; 
but as they expand they become irregularly square, their edges during the free condition 
of the embryo remaining rough with sprouting spicules. 
About the time of the fixing of the pentacrinoid, the basals, which have now assumed 
a somewhat definite form, narrower beneath and expanding above, have their lateral 
edges bounded by straight lines, so that the edges of two adjacent plates are closely 
applied to one another. Even after their edges have become thus defined, the plates 
go on steadily increasing in size, apparently by interstitial growth. The upper edges of 
the basals still remain rounded and rough. Their lower edges are likewise irrregular, 
but these soon become obscured by the growth of the centro-dorsal ring. The oral 
plates extend principally upwards into the oral lobes, where they become lengthened and 
somewhat contracted, their edges fringed with diverging pointed spicules (Plate XXVI. 
fig. 1). . As development proceeds they change somewhat in form. The upper angle is 
slightly depressed, and the sides at the inferior angles are raised, the raised edges at 
that stage lying up against the sides of the second radials. Absorption of the inferior 
portion of the oral plates commences about the time of the appearance of the first 
brachial joints and of the anal plate (Plate XXVII. fig. 1). Both basal and oral plates 
consist at first of a delicate cribriform calcified film, formed by the lateral extension of a 
single layer of calcareous tubing only. As they increase in size, however, they gradually 
thicken, and this thickening is effected by the network sending in from its inner surface 
irregular processes which branch and unite to form a second layer not quite so regular 
as the first, but resembling it in general character. This process is repeated till the 
plates have attained the required thickness. In the oral plates the thickening is very 
slight, and is confined to the lower portion of the plates. 
The stem . — As described above, shortly after the appearance of the spicula indicating 
the basal and oral plates, a chain of six or seven calcareous rings may be observed curving 
from the centre of the space between the bases of the basal plates ; behind, and usually 
somewhat to the left of the pseudostome and pseudocele, and abutting against a round 
