m 
1895. ] NEW YORK ACADEMY OF SCIENCES. 223. 
Nearly all observers agree as to the appearance and location 
of the yolk-nucleus in the egg. It consists of a heap of gran- 
ules of greater or less size lying, at some period of odgenesis, 
near the germinal vesicle, but which may become disseminated 
throughout the egg. A central corpuscle about which the 
granules are arranged in concentric rings has been observed in 
some cases (Siebold (’48), Balbiani (793), Henneguy (’93), but in 
most forms the yolk-nucleus is described as a mere aggregation 
of granules. Its origin has received much attention, but is still 
in doubt, some observers asserting that it arises from the nu- 
cleus and passes into the cytoplasm, while others maintain that 
it originates in the cytoplasm, and in some cases may even mi- 
grate into the nucleus. 
The object of the present preliminary notice is to describe the 
origin and changes of the yolk-nucleus as observed in the de- 
veloping eggs of the earthworm, Lumbricus terrestris. In or- 
der to bring these observations into relation with those of pre- 
vious investigators it is necessary to examine more carefully 
the various contradictory views as to the origin and fate of 
this interesting body. 
The works of various observers may be divided into two 
classes according as the yolk-nucleus is conceived to be of 
cytoplasmic or nuclear origin. 
1. Cytoplasmic origin of the yolk-nucleus. 
Lubbock (’61) regarded the yolk-nucleus as a mere thickening 
of the plasm forming the vitellus, and Balbiani (’66) regarded 
the yolk-nucleus in spiders as arising by budding from the epi- 
thelial tissue of the ovary. This view, however, he subse- 
quently changed. Sabatier (’83) asserted, also in the spider, 
that the yolk-nucleus is formed in the cytoplasm in the 
neighborhood of the germinal vesicle, that it wanders to 
the periphery and there disintegrates. Stuhlmann (’86) re- 
garded it as arising in insects from concretions close to the 
egg periphery and as not identical with the “ maturation 
balls,” which originate at the same time from the nucleus. 
Holl (’90) asserted that in the chick it arises as a mass 
of granules near the germinal vesicle, to which it may send 
out prolongations. Monticelli (’92) claimed that in the case of 
Trematodes there is no connection between germinal vesicle and 
yolk-nucleus and that the latter arises as a protoplasmic differ- 
entiation of the cytoplasm. Jordan (793) came to a similar con- 
elusion in regard to the yolk-nucleus of the newt. 
2. Nuclear origin of the yolk-nucleus. 
The extrusion of nuclear elements has been repeatedly de- 
scribed and by so many authors that it cannot in all cases. 
