No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 507 



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, 

 e.g., 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 ('8l) and Pennington ('97), that the 

 nucleolus in Spirogyra has a true membrane.^ 



A very unusual structure of the nucleolus is that afforded by 

 the salivary gland cells of CJiironomus as described by Balbiani 

 (-81), Leydig ('83), Korschelt ('84), and Macallum (-95). C. 

 Schneider ('9i) 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), 

 Carney and Lebrun ('97a), Will ('85), Holl ('93), Roule ('83), Burger ('90), 

 Ogata ('83), Vejdovsky ('82), Meunier ('86), Carnoy ('86), Mann ('92). 



