Desor on the Embryology of Nemertes. 9 
On turning my attention to the other two yolks, these ques- 
tions were at once answered. Having compressed them so as 
to force them out from the flask, I saw distinctly the three 
zones above described, (fig. 25) ; but the external zone was 
separated from the next zone by an empty space, (a), and I 
could see distinctly that there was an internal motion and 
contraction quite independent of the revolving of the whole 
yolk. On looking more closely, I could see, farther, that the 
margin of the second zone was covered with very minute 
ciliæ, (b). From this moment I no longer doubted that this 
internal motion was made by the animal itself, and that the 
external zone was nothing but an envelop, which is cast off 
by the animal when it approaches the completion of its'em- 
bryonic development. I saw it indeed burst under the pres- 
sure, and fragments of it fall off, as represented in fig. 26. 
In another instance I saw, and had the good fortune to show 
to several of my scientific friends in Boston, the embryo trying 
to escape from this envelop, (fig. 27), which it succeeded in 
doing after a While, and was then seen dragging after it the 
fragments of the external zone, as is represented in fig. 28. 
The various. zones are not only distinct in their external 
appearance, but moreover composed of very different tissues, 
as will be seen by fig. 31, representing, on a large scale, a 
transverse section of the embryo of fig. 27, according to the 
line a b. The outer zone, (m), which is rather dense, is com- 
posed of large cells, apparently irregular, but becoming spher- 
ical on being isolated, (x), each of them having a transparent 
centre. These cells are nothing but divisions of the yolk, as 
I have already described and figured them in fig. 16. Tt 
seems therefore that this part of the yolk has undergone no 
visible change since that epoch, except the appearance of cilize 
on its surface, 
Within this outer coat we find an empty space, narrow, 
but nevertheless sufficiently large to allow the embryo to move. 
Inside of it is seen a transparent zone, (n), covered with cilize, 
this being the real body of the animal. It contains clear cells 
2 oct. 1848. 
JOURNAL B. S. N. H. 
