NUCLEOLAR STRUCTURES OF THE CELL. 155 



Thus, in germinal vesicles of Amphibia, SelacJdi, Sagitta, Meta- 

 nemertini, most Teleostei, and many Arthropoda, frequently hun- 

 dreds are present, and an equally large number is found in 

 clitellar gland cells of Piscicola and in various glands of Arthro- 

 poda. The number in a germinal vesicle is not dependent upon 

 the amount of yolk present in the cytoplasm, since it is the rule 

 in birds and many arthropods that only one or a few occur. 

 There are very few cells in which no nucleoli are present, but 

 that may be the case occasionally in spermatozoa, and in certain 

 connective-tissue elements ; so it would appear that the absence 

 of nucleoli is to be noted mainly in such cells where the nutritive 

 metabolism has ceased or is reduced to a minimum. The num- 

 ber is not constant for all the cells of a given organism, as was 

 claimed by Auerbach. The volume of a nucleolus stands, as 

 a general rule, in inverse ratio to the number ; i.e., at a given 

 stage of a given species of cell the amount of nucleolar sub- 

 stance remains more or less constant. The greatest relative 

 amount of this substance is found in cells which are undergoing 

 rapid growth, and in which, consequently, the nutritive metab- 

 olism is intense ; it is particularly in germinal vesicles, during 

 the growth period, that a large amount of nucleolar substance 

 is found. 



2. General Structure and Position. The ground substance 

 is usually homogeneous, but not infrequently granular (e.g., 

 Gregarines, ova of insects). In most cases it has no limit- 

 ing membrane, the supposed membranes described by various 

 writers being probably in most objects only optical illusions due 

 to light refraction by the curved surface of the nucleolus, and in 

 other cases to the presence of a chromatin reticulum around 

 the nucleolus. Since the nucleolus lies in the meshes of the 

 chromatin reticulum, and is rarely, if ever, penetrated by chro- 

 matin strands, it follows that when a nucleolus is increasing in 

 size it pushes aside the chromatin so that the latter may form 

 a latticework around it. But in a few cases a true membrane, 

 a differentiated portion of the ground substance, is present, as 

 in the germinal vesicle of Polydora. 



The position of the nucleolus is usually excentric, rarely cen- 

 tral. But the position often varies at different stages of the 



