530 
PEOFESSOE W. THOMSON ON THE EMBEYOGENY OF ANTEDON 
in the form of an elongated tubular closed papilla. The summit of the papilla is finally 
absorbed, and a patent anal opening is formed. The details of these later changes 
belong, however, more properly to a subsequent stage. 
Having thus described generally the development of the Pentacrinoid stage of Ante - 
don up to a point when a marked change takes place in its structure and economy, I 
shall now discuss, in somewhat fuller detail, certain general considerations arising from 
the successive steps of the developmental process. 
The relations of the Pseudembryo . — In Antedon the germ-mass is resolved, at all 
events to a great extent, into sarcode having the peculiar delicately vacuolated structure 
so characteristic of this zoological element. The sarcode contains multitudes of “ endo- 
plasts and of oil-cells and granules scattered through its substance, but these latter I 
must regard merely as stores of various organic compounds elaborated as secretions and 
excretions during the development of the organism. In the centre of the sarcode zooid 
there is usually a darker nucleus, indicating a special accumulation of granular matter. 
I have satisfied myself, however, that this condition is not essential, as in some cases in 
which the young were developed in clear water, with a scanty supply of nourishment, 
the pseudembryo became transparent throughout. Still it is conceivable that a germ of 
the original substance of the mulberry-mass may be retained to originate the Crinoidal 
embryo. At all events, the temporary organism which I have termed the Pseudembryo 
is entirely dependent for its form and structure upon the sarcode into which the whole 
or the greater portion of the germ-mass is resolved. This sarcode zooid possesses all the 
peculiarities of the sarcode organisms among the Protozoa and the lower forms of the 
Ccelenterata. Its external surface is richly ciliated, and if lightly touched with a bristle 
it moves off rapidly, by means of these cilia, in a direction opposite to the touch, giving 
-evidence of a high degree of irritability and power of automatic motion, without the 
slightest trace of a special nervous system. During the early stages of its development, 
and before the differentiation of a special assimilative tract, the body increases rapidly 
in size ; the sarcode is therefore capable, as in the case of the astomatous Protozoa, of 
absorption over the whole external surface, and of assimilation throughout the entire 
internal substance. 
Whatever at this stage may be the relations of the granular nucleus of the pseud- 
embryo, I believe the external ciliated absorbent and irritable sheet of sarcode must be 
regarded as a special provisional organ for the nutrition and aeration of the nascent 
embryo. Dr. Carpenter * has already suggested a correspondence between the zooid 
pseudembryo in the Urchins and Starfishes, and the temporary embryonic structures in 
* 44 "We Fere find the yolk-mass converted into a structure, ■which, is destined only to possess a transient 
existence, and which disappears entirely by the time that the development of the offset from it has advanced 
so far that it begins to assume the characters of the permanent organism. This, however, is what takes place 
in the higher vertebrata ; for the structures first developed in the egg of the bird hold nearly the same rela- 
tion to the rudimentary chick, that the 4 Pluteus ’ bears to the incipient Echinus or Ophiura, or the 4 Bipin- 
naria’ to the incipient Starfish.” — Principles of Comparative Physiology, 4th edit. p. 568. 
