1894.] Contributions to the Life-History of the Foraminifera. 155 



V. " Contributions to the Life-History of the Foraminifera.'* 

 By J. J. LISTER, M.A., St. John's College, Cambridge. 

 Communicated by Professor ALFRED NEWTON, F.R.S. Re- 

 ceived May 7, 1894. 



(Abstract.) 



The phenomenon of dimorphism is now known to be presented by 

 many different species of Foraminifera. 



The individuals of a species fall into two groups. In one the 

 central chamber (the Megasphere of Munier-Chalmas and Schlum- 

 berger) is of considerable size, while in the other it is small (Micro- 

 sphere). These two forms of a species may be distinguished as the 

 Megalospheric and Microspheric forms. 



They have been shown to differ, not only in the size of the central 

 chamber, but. in some instances (Miliolidse), in the plan on which 

 the chambers are arranged, in the size attained by the full-grown 

 shell, and also in the frequency of their occurrence, the megalospheric 

 form being much the more abundant. 



It has been suggested that the different conditions under which 

 Orbulina universa is found represent the megalospheric and micro- 

 spheric forms, but the reasons urged in favour of this view appear 

 inconclusive. 



Polystomella crispa (Linn.). 



With the hope of throwing light on the life history of the 

 Foraminifera, a large number of specimens of this species have been 

 examined. 



Like so many others, it is dimorphic. Though the two forms are 

 indistinguishable when the shell is complete, on examining decalcified 

 and stained specimens they may be at once referred to one form OF 

 the other. The central chamber of the megalospheric form is gener- 

 ally about 80 ft in diameter, while that of the microspheric form is 

 about 10^. Associated with the difference in the size of the central 

 chambers there is a marked difference in the nuclei of the two forms. 

 The relative frequency of the megalospheric form to the microspheric- 

 in 1812 examples, is as 34 to 1. 



In the Microspheric form numbers of small nuclei are present, 

 scattered through the protoplasm, but not extending into the terminal 

 chambers. Those in the inner chamber are smaller than those 

 situated further on. The nuclei contain nucleoli of different sizes, 

 lying in an apparently homogeneous internucleolar substance. It is 

 shown that the nuclei increase in number by simple division, and it 

 appears probable that they are so derived from a single nucleus.. 



