The Morphology and Physiology of the Nucleolus. 117 
nucleus. This has been disputed by Gatenby (If), and is not now 
generally believed. When the fertilized egg nucleus divides in 
embryogeny, the “ nucleolo ” is situated at the lower pole of the 
egg. Later it breaks up and certain cells come to contain portions 
of it. These cells, which are situated at the lower pole of the egg, 
give rise to the germ cells of the embryo. 
A somewhat similar germ-cell determinant has been described 
by Haecker in a medusa, Aequorea forskalea (20). About half an 
hour after its egg has been discharged, the nucleolus disappears 
from the nucleus, and a similar body appears in the cytoplasm. 
Haecker believes the latter to be the extruded nucleolus and terms 
it the “ metanucleus.” It persists until the sixty-four cell stage, 
passing intact at each cell division into one cell. It is suggested 
that this cell would give rise to the germ cells. 
Haecker considers the cytoplasmic structures described in eggs 
by several other observers ( f , 6, 2f) to be of a similar nature ; 
probably in these cases also there has been confusion with the 
cytoplasmic organs. 
In Myzostoma, Wheeler (63) describes the nucleolus of the 
oocyte as remaining in the cytoplasm and persisting as a distinct 
body in one of the large posterior macromeres at the eight-cell 
stage. Such a macromere is said to give rise to the endoderm. 
In opposition to these views it should be noted that Gatenby, 
who investigated by the best modern cytological methods, the 
nature of the germ-cell determinant in Apanteles (If), found that 
it originated in the cytoplasm and that neither the cytoplasmic 
organs nor nucleolar, or chromatic emissions from the nucleus, 
were directly concerned in its formation. 
2. The Nucleolus in Somatic Cells. — The term nucleolus is 
applied in the present paper to the so-called “ true nucleolus,” or 
plasm osome. It is usually considered to be a pyrenin body, con- 
taining less phosphorus and more albumen than the chromatin. 
In general it stains with acid stains, while the chromatin takes up 
basic stains. That this is not universally true, has been pointed 
out in previous papers (13, 3f). 
The number of nucleoli occurring in cells varies considerably. 
Prom one to live is the most usual number, but several hundreds 
may be present, as in the subcuticular gland cells of Piscicola. 
Although in most cells a nucleolus is present throughout their 
life, yet in some it develops only in the course of the activities of 
the cells ; the mesenchyme cells of Cerebratulus are an example 
of this (f2). 
Balbiani in 1864 (3) first described movements of the nucleolus 
in living cells, and since that time similar activities have been 
described by many writers for all types of cells, both plant and 
animal. Balbiani described the formation of vacuoles in the 
nucleolus which he said were discharged through a small orifice. 
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