440 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
yolk. In larvae 7 mm. long all there remains of the yolk and periblast is a small nodule 
scarcely larger than the combined bulk of the periblast nuclei which are now all hud- 
dled together (figsl 53 to 54). 
There is no evidence that these cells had any share in forming the blood corpus- 
cles. The yolk at this late stage is almost entirely surrounded by the liver (fig. 54), 
but I think it is erroneous to suppose, as Wilson has done, that the liver finally 
absorbs the yolk. It is still in contact with the sinus venosus and the final absorp- 
tion is accomplished by the blood as truly as the earlier stages of its absorption 
before the liver is formed. In this late stage the yolk no longer shows the yolk cells, 
which are still evident in larvae 5 mm. long. 
Yolk nucleus . — The appearance of this structure at the time of maturity was de- 
scribed under the head of u the mature egg.” In sections of stages with 60 and 72 
nuclei the yolk nucleus consists of a central denser (more deeply stained) portion 
which is surrounded by a thinner substance. In other sections of about the same 
stage the mass is not so divided. In some there are spherical (yolk) bodies scattered 
through it. 
The appearance in many of the stages resembles that of the germinal vesicle of 
the green eggs. In vertical sections the margin is seen to correspond to the sur- 
rounding yolk spheres. Cross-sections of the deeper portions show the same. In a 
somewhat older stage (fig. 32) the outer part of the mass is continued as a thin layer 
over the yolk. 
A peculiar condition is represented by figure 41, in which there is a large 
mass corresponding to the original mass, in which there are scattered larger yolk 
granules. Surrounding this on the yolk side is a deep layer of lighter- stained proto- 
plasm. 
About the time of the closing of the blastopore the yolk nucleus disappears. In 
several eggs in which the blastopore is not yet closed no trace of it can be found, while 
in one egg stained with fuchsine a number of granules are collected in the region 
where this mass of protoplasm was situated. These granules are similar to others 
arranged about the periphery of the yolk, and it is doubtful whether they represent a 
portion of this mass. 
The absorption of this protoplasm must be very rapid, for in but slightly younger 
stages the whole mass is still present. 
Since the above was written, about three years ago, this body, so prominent during 
the early stages of development of the egg, has been identified by one of my students, 
Mr. J. W. Hubbard, with the yolk nucleus, which is a conspicuous body in the ovarian 
eggs from the time they measure 20 ,« to maturity. The yolk nucleus originates as an 
extrusion from the germinal vesicle and reaches the entodermic pole at the time of 
maturity, when the yolk becomes collected about it. 
It was found that during the extrusion of the yolk nucleus the germinal vesicle 
is reduced in size and amount equal to that of the newly formed yolk nucleus. The 
extrusion of nuclear matter which takes place here lends weight to the supposition of 
De Vries and Weismann that definite particles may be extruded from the nucleus 
into the morphoplasm and coutrol it. The functions the yolk nucleus possessed before 
extrusion are retained some time after extrusion in large- yolked eggs, and according 
to the recent theories its functions need not be lost at once, even in those in which its 
