518 
PROFESSOR W. THOMSON ON THE EMBRYOGENY OF ANTEDON 
general integument of the embryo, still retaining sufficiently the histological characters 
of the pseudembryonic integument to leave no doubt that it is simply produced by its 
modification and extension. 
From the appearance of the first traces of the permanent embryonic structures within 
the pseudembryo, the development of the pentacrinoid larva advances' steadily ; and there 
is no natural separation into stages of its subsequent progress until the young Antedon 
drops from the larval stem. At one period, however, during the development of the 
pentacrinoid there is a marked change in the external form and in the anatomical rela- 
tions of the larva, owing to the sudden widening out of the radial portion of the disk, 
and the breaking through of the anal opening. Division of labour has been found 
expedient in the present investigation, and my portion of the task ends just before the 
development of the Pentacrinoid has reached this point. I think it only right, how- 
ever, to mention that Dr. Carpenter, who has been at the same time working out the 
later stages in the development of the Pentacrinoid and the structure of the mature 
Antedon, has most freely communicated to me all his results. My description of the 
development of the pentacrinoid larva has had therefore all the advantage of the light 
thrown upon the earlier stages by Dr. Carpenter’s researches on the later. 
The observations whose combined results have been condensed into the present com- 
munication have extended over the last four years. I have had an opportunity each 
season of watching the more or less favourable development of one or two sets of 
embryos. As stated above, these observations have not in all cases thoroughly tallied ; 
their inconsistencies depending, I believe, in some instances upon error of observation, 
and in others upon actual discrepancies in the process of development under different 
circumstances. In Arran, in June 1860, I had a most favourable opportunity of tracing 
a single brood from the segmentation of the yelk almost to the maturity of the penta- 
crinoid young. I took the opportunity to revise and check previous special observa- 
tions ; and each stage of the development of this group was described and figured with 
great care, and with the advantage of previous familiarity with the successive modifica- 
tions in form. To avoid all possibility of confusion, I have incorporated in the following 
detailed description those results only which were confirmed by these later observations ; 
and all the figures of the free pseudembryos, and of the origin of the pentacrinoid form, 
refer to the successive stages in the development of this single brood. On this occasion 
the pseudembryos remained for perhaps a somewhat shorter time than usual in their 
free condition, and their growth was early arrested by the development of the perma- 
nent calcareous plates. The pseudembryos, however, during their brief independent 
existence, attained their perfect and usual external form ; and the subsequent transitions, 
though rapid, were normal. 
The ovaries of Antedon have been frequently described. During the latter part of 
summer, autumn, and early winter they can only be traced as delicate lines of whitish 
stroma, beneath the integument of the upper (oral) surface of the pinnules, and imme- 
diately beneath the tentacular canals which in the ordinary condition of the pinnules 
