106 RAYMOND PEARL AND MAYNIE R. CURTIS 
Crucial evidence is here afforded by those cases occasionally 
to be observed, where the egg is just entering the isthmus, and 
has one end in the albumen portion of the duct and the other end 
in the isthmus. It was first pointed out by Coste?’ that the forma- 
tion of the shell membrane at the upper end of the isthmus is a 
discrete process. That is, as the end of the egg advances from 
the albumen portion into the isthmus, membrane is deposited 
upon it. The membrane is complete over the whole egg only 
after the egg has entirely passed into the isthmus. This account 
of membrane deposition we have confirmed by direct observation 
in this laboratory. Now in cases where one-half of the egg lies 
within the isthmus and bears a membrane while the other half 
is in the albumen portion and has no membrane it can plainly 
be seen that the shell membrane is deposited directly on the outer 
surface of the thick albumen (layer C) and that no trace of the thin 
albumen (layer D) is present at the time the membrane is formed. 
It might be contended that the thin albumen which is to form 
layer D is really present at the time the membrane is deposited, 
but that instead of forming a separate outer layer it is held by 
adhesion or otherwise within the meshes of the fibrous network 
of the dense albumen of layer C. On this view it might be sup- 
posed that this more fluid albumen passes out of the network to 
form a definite and separate layer at some time after the mem- 
brane is laid down. This contention, however, cannot be correct, 
because, as will be demonstrated in the next section of the paper, 
the egg does not have its full complement of albumen by weight 
at the time when the shell membrane is formed. The fluid 
albumen of layer D should weigh just as much, whether in the 
interstices of a fibrous meshwork, or forming a separate layer. 
Yet the facts show that after a thin albumen (layer D) has been 
visibly formed the egg contains by weight about 50 per cent more 
albumen than it did before this layer was visibly formed. 
8 Coste, M., Histoire du développement des corps organisés, tome 1, p. 295, 1847. 
