424 
MONTGOMERY. 
[Vol. XV. 
the primitive yolk ball may break either into two fragments 
(which are usually unequal in dimensions), each of which then 
fragments further, or it breaks at once into a considerable 
number of larger granules. The final stage in this process of 
division shows the daughter yoke balls fragmenting to form 
the ultimate yolk spherules (Fig. 118) ; the latter stain an 
orange red with eosin, are homogeneous in appearance, and 
usually oval or spherical in form, seldom irregular. Two main 
stages may accordingly be distinguished in the formation of the 
yolk : (i) the formation of a large, regularly shaped yolk ball ; 
and (2) the successive fragmentation of this ball, accompanied 
by a gradually lessening affinity for stains, resulting in the 
evolution of the mature, small yolk spherules, the cytoplasm 
of the ripe egg being thickly filled with the latter. It is usu¬ 
ally the case that the yolk ball attains its greatest size at the 
end of the first stage. In cells of medium size all the various 
stages of yolk formation may be found, which shows that the 
yolk balls are being successively produced and are successively 
fragmenting ; quite a number of these balls need to be pro¬ 
duced in order to furnish the large quantity of yolk globules of 
the mature egg. The time when the yolk balls first appear, 
the size they reach, and the manner in which they segment, 
seem to vary much in individual cells. 
I have not been able to determine the manner of the first 
differentiation of the yolk substance in the cytoplasm. Two 
possible explanations suggest themselves : (1) either a certain 
portion or constituent of the cytoplasm changes into yolk sub¬ 
stance ; or (2) the yolk balls may represent a nutritive 
substance accumulated in the cytoplasm, which may have been 
derived from the blood or from some neighboring tissue, if not 
directly from the posterior intestine. But it is without doubt 
that this substance is not of nuclear origin, for the yolk balls at 
their first appearance are not in contact with the nucleus, but 
usually at some distance from it ; and also during the earlier 
stages of the yolk formation the nucleus is irregular in outline, 
with short, blunt processes, which would show that it is tak¬ 
ing up substances from the cytoplasm, rather than excreting 
substances. 
