Desor on the Development of Polynée. 13 
who has taken up the history of these animals from the first 
period of their development, showing that at the spawning 
season, the eggs fill nearly the whole cavity of the. animal, 
being of a purplish tint, and showing distinctly the germinative 
vesicle. When laid, the eggs are found deposited under the 
scales of the back, where they are kept in place by an adher- 
ing membrane. "There they pass through the first stages of 
their growth, namely, the subdivision of the yolk, which is fol- 
lowed by the first appearance of the germ, both of which take 
place in a very regular manner. After a while, cili& appear 
on the surface of the yolk, which enable the eggs to move by 
jerks. "These cilie grow rapidly, while the embryo changes 
its color from pink to green. When they have arrived at 
this stage, the embryos leave their retreat simultaneously, and 
begin to swim freely in the water. 
It is at this period that I found, last February, thousands of 
litle green spots moving with great rapidity in a jar, where 
there were several .Polynóe kept. Having examined them 
with the microscope, I was struck, like M. Sars, to see that 
they had not the least resemblance to their parents. They 
. were small spherical bodies, surrounded with a ring of long 
threads, (fig. 36,) by means of which they spun round, all 
the time changing their places, sometimes starting in a straight 
line in one direction, sometimes describing parabolic lines, 
and then stopping short and spinning round like tops.* 
Sometimes also, instead of spinning on their own axes, they 
turned somersets, and then it was easy to perceive that the 
threads formed a regular ring, (fig. 37.) When the spheres 
spun around, I saw at the base of each thread an enlargement, 
which is simply a fold of the membrane to which the threads 
are attached, as is plainly shown in the enlarged sketch of 
fig. 39. The movement of the threads resembles that of a 
whip-lash, the middle part bending first. 
I was surprised to find such rapid and extraordinary motion 
in the progeny of an Annelid, and though it appeared quite 
! Wiegman, Archiv. 1836. ? Wiegman, Archiv. 1846. 
