Huxley on Lacinularia socialis. 
11 
The ova are developed thus:— One of the vesicles in- 
creases in size, and reddish elementary granules appear in 
the homogeneous substance round it (fig. 10). This accumu- 
lation increases until the ovum stands out from the surface 
of the ovary ; but invested by its membrane which, as the 
ovum becomes pinched off as it were, takes the place of a 
vitellary membrane. 
In the mean while the germinal vesicle has increased in 
size, and its nucleus is no longer visible. In the ovum it 
appears as a clear space ; isolated by crushing the ovum it is 
a transparent, colourless vesicle. 
The perfect ova are oval, about 1-lOth inch in diameter, 
and are extruded by the parent into the gelatinous connect- 
ing substance, where they undergo their development (fig. 11). 
The changes which take place after extrusion, or even to 
some extent within the parent, are — I, the disappearance of 
the germinal vesicle (as I judge from one or two ova in 
which I could find none) ; 2, the total division of the yolk, 
as described by KoUiker in Megalotrocha^ until the embryo 
is a mere mass of cells, from which the various organs of the 
foetus are developed (figs. 12, 13, 14, 15, 16). 
The youngest foetuses are about l-70th of an inch in length. 
The head is abruptly truncated, and separated by a con- 
striction from the body : a sudden narrowing separates the 
other extremity of the body from the peduncle, which is ex- 
ceedingly short and provided with a ciliated cavity, a sort of 
sucker, at its extremity. The head is nearly circular, seen 
from above, and presents a central protuberance in which 
the two eyespots are situated. The margins of this pro- 
tuberance are provided with long cilia — it will become the 
upper circlet of cilia in the adult. 
The margin of the head projects beyond this, and is fringed 
with a circlet of shorter cilia ; this is the rudiment of the 
lower circlet of cilia in the adult. The internal organs are 
perceived with difficulty ; but the three divisions of the ali- 
mentary canal, which is as yet straight and terminates in a 
transparent cloaca, may be readily made out. The water- 
vascular canals cannot be seen, but their presence is indicated 
by the movement of their contained cilia here and there 
(fig. 17). 
In young LacinularicB, l-30th of an inch in length, the head 
has become triangular, the peduncle is much elongated, and 
thus it gradually takes on the perfect form (fig. 18). The 
young had previously crept about in the gelatinous investment 
of the parents ; they now begin to " swarm," uniting together 
by their caudal extremities, and are readily pressed out as 
