No. 2.] 
COMPARATIVE CYTOLOGICAL STUDIES. 
5 ° 7 
4. General Morphological Structure of the Nucleolus. 
The ground substance of the nucleolus is more or less dense, 
but not brittle, and either homogeneous or finely granular, 
rarely coarsely granular. It may be either * fluid or viscid in 
consistency. 
In the greater number of cases it has no limiting membrane. 
Such a membrane was found by me only in the germinal spot 
of Polydora , and here it appeared to be merely a denser portion 
of the ground substance. When any small nucleolus is viewed 
in its totality a membrane appears to surround it, but this 
phenomenon is due to the refraction of light from its convex 
surface, and many observers have been misled by this appear¬ 
ance into supposing that a membrane is present. Others in 
describing those states of nucleoli in which a large vacuole is 
present have erroneously described the peripheral layer of true 
ground substance as a nucleolar membrane ; it is necessary to 
distinguish between such a peripheral layer, which consists of 
true ground substance, and a nucleolar membrane proper, which 
is a differentiation of the ground substance. Some authors, 
eg., Lavdowsky (’94), have described a membrane of chromatin 
enveloping the nucleolus, and I have found that those of the 
giant cells of Doto may sometimes be surrounded by a mass of 
chromatin. But this apposition of a mass of chromatin in Doto 
is certainly an artefact, though it would seem probable that 
the nucleolus in some cases has an envelope of chromatin 
forming a distinct capsule separated from the chromatin net¬ 
work of the nucleus. I am able, however, to corroborate the 
observations of Macfarlane (’8i) and Pennington (’ 97 ), that the 
nucleolus in Spirogyra has a true membrane. 1 
A very unusual structure of the nucleolus is that afforded by 
the salivary gland cells of Chironomus as described by Balbiani 
(’8i), Leydig (’83), Korschelt (’84), and Macallum (’ 95 ). C. 
Schneider (’ 91 ) supposes the nucleoli, as well as the rest of the 
nuclear substance, to consist of “ Geriist ” (linin ?) and chro- 
1 The following writers have described nucleolar membranes: Macallum (’95), 
Carnoy and Lebrun (’97a), Will (’85), Holl (’93), Roule (’83), Burger (’90), 
Ogata (’83), Vejdovsky (’82), Meunier (’86), Carnoy (’86), Mann (’92). 
