138 J. J. LISTER. 



into the succeeding chamber, as though fixed in the act of pass- 

 ing from one chamber to another (figs. 13, 15, and 23). 



Position of the Nucleus. — As Schulze has shown (42), the 

 nucleus usually lies in or somewhere near the chamber which is 

 numerically in the middle of the series from the megalosphere 

 to the terminal chamber. There is some evidence to show that 

 its position depends, in part at least, on the disposition of the 

 bulk of the protoplasm. This is most clearly seen in young 

 specimens. It has been pointed out above that the megalo- 

 sphere varies considerably in size in different specimens. Now 

 in specimens with few chambers the megalosphere, being con- 

 siderably larger than the chambers which immediately succeed 

 it, contains a considerable quantity of the whole bulk of the 

 protoplasm. If in a young specimen the megalosphere is large, 

 the nucleus is found to be less advanced along the series of 

 chambers than in those in which the megalosphere is small. In 

 such specimens it appears that the nucleus tends to lie near the 

 centre of the protoplasm. 



In specimens with twenty to thirty chambers, however, the 

 nucleus is generally found, as above stated, near the middle of 

 the series, and as the chambers which follow the megalosphere 

 successively increase in size, it is clear that its position is some 

 distance behind the centre of the whole of the protoplasm. 



As will appear below, in dealing with other species whose 

 growth is spiral {Rotalia, Calcarina, Truncatulina) the nucleus 

 in like manner moves on from chamber to chamber as the 

 number of chambers increases. In Orbitolites and Gycloclypeus, 

 on the other hand, whose growth is cyclical, the nucleus in the 

 megalospheric form remains in or close to the first formed 

 chamber, which, owing to the mode of growth, is at the centre 

 of the protoplasm. 



The Size of the Nucleus. — The nucleus increases in size as the 

 protoplasm grows. The truth of this statement may be realised 

 by inspecting the accompanying diagram (p. 140), in which the 

 results of the measurement of a number of examples are given. 

 The abscissae indicate the diameter of the nucleus in /t's, and 



