Desor on the Embryology of Nemertes. il 
enter into the vitellus, (fig. 29.) A few days later, the yolk 
residue is nearly transparent ; the part of the intestine seen in 
it generally appears bent ; and, besides this, there is seen near 
the posterior extremity a large clear spot, which indicates pro- 
bably the anus, (fig. 30.) 
My observations were here brought to a close, it being im- 
possible to pursue the further development of the young 
Nemertes, when escaped from the flasks, in consequence of 
their small size, and the difficulty of preserving them alive. 
RECAPITULATION. 
The main point in these researches is the fact, that a large 
portion of the yolk is transformed into an envelop, which sur- 
rounds the embryo during the first phases of its existence, and 
which is then cast off by it when it becomes able to move by 
itself. This liberation of the embryo from the envelop must not 
be confounded with the casting off of a mere external mem- 
brane, like the shell membrane, or like the placenta of the 
mammalia. Indeed, the placenta, as we know, is formed of a 
combination of the chorion or vitelline membrane with the 
maternal organs, The envelop of which we speak is not the 
product of the vitelline membrane ; it is an integral part of 
the yolk itself. 
The liberation fror this envelop is by no means a substi- 
tute for the process of hatching, which takes place as regu- 
larly in the Nemertes as in any other animal. Indeed the 
embryo which throws off its envelop is not hatched by this 
process. We have shown, on the contrary, that. it continues 
to remain for some time after its liberation in the common 
egg, which has been designated under the name of flask, 
anditis only when leaving it, that we can consider itas hatched. 
Consequently the presence of an envelop forming an 
! One might be induced to compare this Mene from the envelop with the 
moulting of caterpillars; but it is to be remembered, that in moulting, it is 
the skin that is concerned ; that is to o say, a iie of the animal composed of or- 
ganized tissue, whilst the euvelop of the Nemertes has not yet arrived at this state, 
but seems to be merely yolk substance. 
