45 
much more marked, and the first micromere c has also begun 
to unite with the macromere. Up to this time the lines of' 
union of the three spherules have been visible, but in another 
minute, Figure 10, there is no line to indicate the fusion of 
the second micromere b with the macromere a , and the prim- 
itive distinctness of the two is only indicated by a depression 
in the outline, which soon disappears entirely, as is shown in 
Figures 11 and 12. At the same time the first micromere, c y 
becomes more completely united to the mass, a and 5, formed 
by the fusion of the second with the macromere, although, a & 
shown in Figures 11, 12, 13, 14 and 15, the line of separation 
never becomes invisible, Of these, Figure 11 is three minutes- 
later than Figure 10, and Figure 12 two and three-quarters 
minutes later than that. The next figure, 13, is from another 
egg, but is about two minutes later than 11, and shows the 
characteristics at the end of the second period of rest. At 
some time during the stages of which I have been speaking 
the egg sheds an external membrane, and I have copied this 
drawing here in order to show the membrane, nearly cast off. 
It will be seen that the casting of this membrane does not 
leave the surface of the yolk exposed, but that it is still cov- 
ered with an investing membrane. I am unable to say 
whether the membrane was originally more than one layer 
thick, or whether a new one is formed to replace the one 
which is shed. The time wdien the egg escapes from this 
membrane varies greatly, but it is usually earlier than this 
stage, and an egg at the stage 10, in the act of escaping, is 
shown in Figure 46. At the stage 13 the egg is again almost 
spherical, and consists of two masses, a large one, a and b , and 
a small one, c, meeting each other upon a fiat surface. The 
preceding stages show that the larger mass is compound, and 
made up of the macromere and second micromere, but there 
is no visible indication of this fact. The long axis of the egg,, 
at this stage, does not pass through the polar globule, but 
parallel to it. 
At the stage which corresponds to this the eggs of many 
molluscs have a segmentation cavity, but there does not seem 
to be any space between the two spherules of the oyster-egg* 
