A Comparative Study of the Structure and Origin of the Yolk Nucleus. 687 
cspecially that of the tortoise, it owes its prominence to an infiltration, 
between the constituent fibers, of karyolvmph; which, wlien coagulated 
by reagents, makes it look more like a homogeneous mass. Consequently 
the finer details visible, for instance in some eggs of the kingcrab, are 
less evident. Possiblv other methods of preparation, than those I have 
tried might bring the finer details into view. 
It is conceivable that, when this archoplasm, partly or wholly sur- 
rounding the germinal vesicle, becomes vacuolated by the formation of 
metaplasm, it would be converted into a network enclosing granules. 
It would then resemble that broad granulär ring around the germinal 
vesicle, so often seen in the eggs of fishes. The sphere would then be 
obscured and possibly be invisible altogether. 
As in the other eggs, so in this, there is an axis differing from all other 
axes, that, namely, connecting the vitelline body (sphere) and the germinal 
vesicle. That is also indicated by the indentation of the germinal vesicle, 
where it is in contact with the sphere. 
The Yolk Nucleus in Egg of Crayfish. 
Of all the eggs examined, the crayfish egg, seems on a superficial ex- 
amination, least likely to reveal any trace of a yolk nucleus, such as I 
have described in other eggs. The larger eggs seem to be radially svm- 
metrical. The germinal vesicle is usually surrounded by a ring of less 
granulär protoplasm, which sends out processes into the surrounding yolk. 
But in many younger eggs, an indentation of the nucleus at one pole 
is visible. This indentation is often so marked as to give the germinal 
vesicle the appearance of a crescent, and occasionally the form of a horse- 
shoe, fig. 77. 
The finelv granulär protoplasm, surrounding the nucleus, fills the 
cavitv in the nucleus. Owing to the extreme minuteness of the fibers 
and the granules in this part of the cell, with all those methods that I 
have emploved, it is difficult to make out any definite structure. 
But when the indentation is not so pronounced, as where there is only 
a notch in the otherwise circular germinal vesicle, a more Condensed 
portion can be detected in that notch. A slightly lighter circular vesicle 
can be seen in the center of this, often close to the nuclear membrane, 
as in fig. 75. Surrounding it, are distinct indications of concentric circles, 
sometimes visible only part way around. 
Surrounding this again is a large body of undifferentiated protoplasm, 
conforming to the outlines of the germinal vesicle. This taken in con- 
junction with the latter, completes an oval area occupying the center 
