214 GERM-CELL CYCLE IN ANIMALS 



cleoli, consisting of usually a single principal nucleolus 

 (Hauptnucleolus of Flemming), and one or more 

 accessory nucleoli (Nebennucleoli of Flemming). 

 Many nucleoli have been described that may perhaps 

 represent intermediate stages in the evolution of one 

 of the types mentioned above into another. 



The young ovarian egg of most animals contains a 

 single spherical nucleolus ("Keimfleck," or "germi- 

 nal spot"), but the number may increase greatly dur- 

 ing the growth period. Usually during the formation 

 of the first maturation spindle the nucleolus escapes 

 from the nucleus into the cytoplasm, where it dis- 

 appears, often after breaking up into fragments. 

 Many theories have been advanced regarding the 

 origin, function, and fate of the nucleoli of the germi- 

 nal vesicle. They are considered by some of chro- 

 matic origin, arising as an accumulation of the chro- 

 matin, or from the chromatin by chemical trans- 

 formation. Others consider them extra-nuclear in 

 origin (Montgomery, 1899). 



Many functions have been attributed to the nu- 

 cleoli ; of these the following may be mentioned : 

 (1) They function as excretory organs (Balbiani, 

 1864 ; Hodge, 1894) ; (2) nucleoli play an active 

 role in the cell, since they serve as storehouses of 

 material which is contributed to the formation of the 

 chromosomes (Flemming, 1882 ; Lubosch, 1902 ; 

 Jordan, 1910; Foot and Strobell, 1912) and may 

 give rise to kinoplasm (Strasburger, 1895) or *'Kine- 

 tochromidien " (Schaxel, 1910); (3) nucleoH are 

 passive by-products of chromatic activity; they 



