492 Mr. Joseph Jackson Lwter [Feb. 15, 



Isle of Wight, in which they are of nearly the same size, was here 

 displayed.] 



The names megalosphere and microsphere have been given to the 

 large and small initial chambers respectively, and the two forms are 

 known as mef/alosjjheric and mkrosijheric. 



The examination of other groups of Foraminifera has abundantly 

 confirmed Munier-Chalmas' view as to the general presence of dimor- 

 phism. 



We now turn from fossils to the life-history of PolystomeUa crispa 

 and inquire what light we may gain from it on the significance of the 

 phenomenon of dimorphism. 



In a large batch of Polystomellas, killed by a reagent which dis- 

 solves the calcareous shell, it will be found that megalospheric and 

 microspheric individuals occur in the batch, as among the nummu- 

 lites of the Eocene strata. 



On staining them, another point of difference between them 

 becomes apparent. The megalospheric form is uninucleate, the 

 microspheric form multinucleate. 



[Diagrams of the megalospheric and microspheric forms of Poly- 

 stomeUa crispa, with the shell removed and nuclei stained, were here 

 shown.] 



During the growth of the microspheric form, the nuclei multiply 

 by simple division. They also give off small irregular strands into 

 the protoplasm. These have been called chromidia. Eventually, 

 prior to reproduction, all the nuclear material appears to be resolved 

 into chromidia. 



In a culture of PolystomeUa it is common to find a mode of 

 reproduction which on examination will be found to be that of the 

 microspheric form. It is best followed when occurring in a specimen 

 attached to a glass slide. Such specimens are, in the early phases, 

 distinguished by a great increase in the number of pseudopodia 

 issuing from the shell, so that the latter a])pears when seen by trans- 

 mitted light to be surrounded by a milky halo. The protoplasm 

 gradually emerges from the shell until, after some hours, the whole 

 of it has come out and lies massed between the shell and the sup- 

 porting surface and within the area formerly covered by the halo. 

 The internal ])rotoplasm is darkly coloured with brown granules, and 

 the whole mass is during this time the seat of involved streaming 

 movements. Clear spots make their appearance, and gradually the 

 protoplasm collects about these, and se})arates into as many spherical 

 masses, which remain connected by a felt of hyaline pseudopodia. 

 Some 200 is a common nund)er to be found. Not long after they 

 have become distinct it may be noticed that each attains a shining 

 coat — the indication that a shell has l)een formed, a small apei'ture 

 being left in each foi* the passage of the ])seudo])odia. After lying in 

 close contact for some hours, the spheres rajmlly and sinudtaneously 

 draw apart from one another, and within half an hour from the 



