514 
PROFESS OR W. THOMSON ON THE EMBRYOGENY OE ANTEDON 
in his own ‘ Beobachtungen iiber Anatomie und Entwickelung einiger wirbellosen 
Seethiere’ (Berlin, 1851). 
The author alludes to the position of the ovary in Antedon , and to the peculiar way 
in which the impregnated ova remain hanging in bunches from the ovarian aperture. 
He describes the formation from the segmented yelk-mass of a uniformly ciliated club- 
shaped embryo, which escapes from the vitelline membrane and swims freely in the 
water (Beobachtungen, &c., pi. 13. fig. 13). During the next four-and-twenty hours a 
bunch of long cilia appears on the narrower anterior extremity, and near it, on the side 
of the embryo which is turned downwards in a state of rest, a small round opening which 
he regards as the provisional larval mouth. Three slightly elevated ridges now gird the 
body transversely at equal distances (op. tit. pi. 13. fig. 14), and gradually become clothed 
with long cilia, the smaller cilia disappearing from the intervening spaces. The inte- 
gument between the first and third ciliated ring becomes inverted into a large oval 
depression, a fourth ciliated band appears near the posterior extremity of the embryo, 
and a few delicate areolated calcareous plates are developed within the integument. 
The embryo now becomes slightly curved, the large oval opening which the author 
regards as the excretory orifice becomes more distinct in the centre of the ventral surface, 
and the embryo attains its most perfect larval form (pi. 14. figs. 1 & 2). The form of 
the larva now rapidly alters ; on the ninth day (pi. 14. fig. 3) the posterior extremity has 
become much enlarged and invested with a thick gelatinous integument. This distended 
extremity becomes slightly lobed, the anterior bunch of cilia and the posterior ciliated 
bands disappear, the mouth and anus become indistinct (pi. 14. fig. 5), and at length 
(pi. 14. fig. 6) a row of four delicate tubes bearing pinnules appears along either side of 
the larva, the rudiments of the arms of the Crinoid. Dr. Busch was unable to pursue 
his researches further. In many points his observations are inconsistent with those 
which I have repeated during the last three years with great care, and I believe that 
he has misconceived the nature and relations of the organs of the larval embryo. Dr. 
Busch’s account of the first appearance of the pentacrinoid form is certainly contrary to 
my experience ; I have been led, however, by inconsistencies in my own observations 
upon different broods in different seasons, to believe that the mode of development may 
to a certain extent vary with circumstances. I find, for instance, that when the ova are 
liberally supplied with fresh sea-water and placed in a warm temperature, the later 
stages of larval growth are, as it were, hurried over ; so that the free larva scarcely 
attains its perfect form before being distorted by the growing crinoid. In other 
instances, in colder seasons and in a less favourable medium, the larva reaches a 
much higher degree of independent development, and retains for a longer period the 
larval form. 
In 1859 I communicated to this Society a short notice (Proc. Boyal Society, vol. ix. 
p. 600) of the earlier stages in the development of Antedon. My observations were made 
upon one or two broods of Antedon in a single season. I had an opportunity at that 
time of tracing carefully the earliest phases in the development of the pseudembryo, but 
