he 
Desor on, the Embryology of Nemertes. 7 
and consequently we see, in most of the flasks, small bub- 
bles floating about, as has been observed in many mol- 
lusks, and even in mammals. These bubbles seem to preserve 
a kind of separate life; at least, they do not decompose, an 
we find them even in those flasks whose embryos are far 
advanced, (fig. 18 and 20.) They are always surrounded by 
a distinct membrane, and contain a certain number of small - 
granules; but I never saw them assuming any other form 
than that of simple vesicles, destitute of cilie. 
At about the fourteenth day the yolks begin to move.' 
When observed with a high power, I found them covered 
with very minute cilie, their organs of motion, (fig. 17.) 
They move at first very slowly and irregularly, revolving about |. 
their own centre. "The liquid in which they revolve does not 
seem to offer much resistance, from the apparent ease with 
which they push about the little bubbles contained in their 
enclosure with them. When a flask is broken and the yolks 
have escaped, they continue to agitate their cilie, and to move 
as well in the water as they did in the liquid contained in the 
flask. This is a sufficient proof that this motion depends 
upon inherent power, and is not the result of mere external 
influences, such as difference of density. 
About this time the yolks, which until now had seemed 
quite homogeneous, begin to change their appearance. We 
perceive in the interior a very transparent spot, lying trans- 
versely, and which must not be confounded with the clear 
spots heretofore spoken of, from which they may easily be 
distinguished by their peculiar shape and sharper outlines, 
(fig. 18.) Upon the application of pressure, the crescent, 
becomes larger, and very distinct, (fig. 19.) — 
We now also perceive two separate zones in the embryo, 
the external one being clear, and the internal more opaque. 
The external zone becomes more and more distinct, so as to 
abet one flask, taken from another mass of eggs, I found motion by the twelfth 
