520 
PROFESSOR W. THOMSON ON THE EMBRY 0 GrENY OF ANTEDON 
cavity of the pinnule is filled with a mucilaginous liquid charged with myriads of mature 
spermatozoa (Plate XXIII figs. 5 & 6). 
The form of the spermatozoon is intermediate between that of a club on cards and a 
spade (Plate XXIII. fig. 7), with a vibratile filament of great length attached to the 
obtuse end. There is no special opening to the testis, so that the female may be at 
once distinguished by the ovarian aperture. The seminal fluid seems to be discharged 
by the thinning away and dehiscence of the integument. The spermatozoa are dispersed 
in the water. Impregnation appears to take place after the discharge of the ova, but 
while they are still hanging from the ovarian aperture. 
An hour or two after impregnation the germinal vesicle disappears, or at all events 
leaves its former superficial position. The yelk-mass contracts and becomes more opaque 
and dense, leaving a clear space immediately within the vitelline membrane, which is 
thus more clearly defined, perfectly transparent and structureless, with the surface 
slightly and irregularly echinated (Plate XXIII. fig. 8). Consequently on the con- 
traction of the yelk, a number of minute spherical pale yellow oil-globules are appa- 
rently pressed out into the space within the vitelline membrane (Plate XXIII. fig. 11). 
The appearance of the “ richtungs-blaschen ” may be very readily traced in the egg of 
Antedon. At a point on the circumference of the yelk a very distinct globule, about 
half the diameter of the germinal vesicle, with an obscure nucleus, passes out of the 
yelk-mass into the surrounding space. In all the cases in which I have observed it, 
this globule has been accompanied by two or three minute rounded granular masses. 
Plate XXIII. fig. 14, a-c, are careful representations of three groups of these globules. 
They remain perfectly distinct from the divisions of the yelk during the earlier stages 
of segmentation ; at the close of this process, however, it becomes difficult to distinguish 
them from the ultimate divisions of the mulberry mass. In Antedon , yelk-segmentation 
is complete (Plate XXIII. figs. 9-13). Its first appearance is a slight groove passing- 
inwards from the circumference of the yelk, immediately at the point where the so-called 
“richtungs-blaschen” have been extruded. If the egg be now subjected to slight 
pressure, a transparent nucleus may be observed in the centre ; and at each stage of 
segmentation the nucleus may be readily detected in the centre of each segment. A few 
hours after segmentation has been completed, the surface of the germ-mass becomes 
slightly more transparent. The ultimate yelk-spherules are still sufficiently evident, 
giving the surface a distinctly mammillated appearance (woodcut A). 
This gradually disappears, the spherules seem to coalesce upon the outer surface, 
remaining distinct a little longer towards the inner surface of this rudimentary germinal 
membrane, and a few hours later they have become entirely fused into a continuous 
structureless sarcode-layer (woodcut B). While these changes are taking place in the 
outer layer, the central portion of the germ-mass becomes resolved into a mucilaginous 
protoplasm sufficiently fluid towards the centre to allow of an active circulation of 
granules and oil-globules, but apparently continuous with, and graduating into, the 
lower surface of the more consistent peripheral layer. 
