532 
PKOEESSOR W. THOMSON ON THE EMBRYOGENY OE ANTEDON 
embryonic nurses, presenting a distinct bilateral symmetry in the arrangement of their 
alimentary system and natatory apparatus. A certain community of plan appears to 
run through the swimming group described by Professor Muller ; but subsequent 
observations would seem to indicate that so high a development of the pseudembryo is 
exceptional. 
In genera closely approximated to those in which the pseudembryo is most highly 
organized, or even in allied species of the same genus, the pseudembryonic appendage 
is reduced to a mere rudimentary vascular tuft, or to a simple investment of sarcode. 
My own observations would lead me to suspect that the independent development of 
the pseudembryo may be greatly modified, even in the same species, under different 
circumstances of light, warmth, aeration, and nourishment. 
The pseudembryo of Antedon resembles very closely what Professor Muller has 
described as the “ pupa stage ” in certain Holothuridea. The young Holothuria, how- 
ever, has in these instances, according to Muller’s observations, passed through the 
phase of a pseudembryonic zooid (Auricularia), with a special mouth and alimentary 
canal, special natatory lobes, and a regular bilateral symmetry, before assuming the 
pupa form of a closed sarcode-cylinder girded with ciliated bands and devoid of special 
organs. In Antedon the “Auricularia” and the “pupa” stages are, as it were, fused 
into one. The “pupa” form is at once developed from the germ-mass, but it is pro- 
vided with the assimilative organs of the Auricularia, though in a very rudimentary 
degree. Further metamorphosis proceeds very similarly in both cases. In both the 
organs of the young are gradually differentiated within a sarcode-cylinder, the branchial 
tentacles finally protruding through an anterior sarcode dome. The close analogy is 
highly marked in the Synaptidse, the group whose metamorphoses have been observed 
by Muller, in which, as in the Crinoids, the oral tentacles are highly developed at the 
expense of the vessels of the ambulacral region. One or two remarkable differences, 
however, exist. In Antedon no part whatever of the alimentary canal is adopted by the 
nascent Crinoid. In Antedon the development of the organs of the embryo is confined 
to the anterior region of the pseudembryo, the posterior portion containing the stalk, a 
temporary appendage. In the Holothuridea the whole pupa passes by simple metamor- 
phosis into the body of the perfect form, the apical pole being occupied by the excre- 
tory orifice of the alimentary canal. In the Holothuridea the madreporic tubercle and 
the sand canal, though frequently extremely rudimentary in the mature form, seem 
uniformly conspicuous during the development of the young. In the pseudembryonic 
stage of Antedon no trace of this organ has been observed. 
I believe that, in zoological language, the term “ embryo ” has hitherto been under- 
stood to indicate a young animal during the early stages of its development ; an orga- 
nism which is produced by the differentiation of the whole or of part of the segmented 
yelk, and which is a stage in progress towards the mature form of its species. Any 
accessory or deciduous parts have usually been termed embryonic appendages ; but these 
embryonic appendages have always been regarded as parts of the embryo, although 
