238 Mr Lister, The young of the microspheric forms [Jan. 25, 



plane dividing any single chamber symmetrically, is not identical 

 with the corresponding plane of the preceding chamber but 

 directed at a definite angle to it. It is as though in a Biloculina 

 shell, while the plane in which the new chambers were formed 

 remained constant, the long axis were rotated through a definite 

 angle in the interval between the formation of each chamber and 

 the completion of its predecessor. The result is that the walls of 

 several chambers take part in forming the external contour of the 

 shell. In Quinqneloculina five chambers take part, at any given 

 stage of growth in forming the contour. In Triloculina where the 

 " rotation of the axis " is through a different angle the contour 

 presented by the shell is formed by three chambers. 



In the microspheric form of the genus Biloculina the arrange- 

 ment of the chambers which succeed the microsphere is at first on 

 the quinqueloculine plan, then on the triloculine, and it is not till 

 several chambers have been formed that the biloculine plan is 

 attained. 



Similarly M. Schlumberger has shown that in the genus 

 Triloculina the chambers are at first arranged on the quinque- 

 loculine plan in the microspheric form, the triloculine plan being 

 attained later. 



What is the explanation of this remarkable change in the 

 mode of growth in the microspheric form of Biloculina and Trilo- 

 culina ? And why should it be characteristic of the microspheric 

 form, while the megalospheric form attains the arrangement of 

 chambers characteristic of the genus to which it belongs from the 

 beginning of its growth ? 



It appears possible that the explanation is to be found from 

 the consideration of the different ways in which the two forms are 

 produced. 



In dealing with the life history of Polystomella I have shown 

 that there is good reason to regard the megalospheric and micro- 

 spheric forms of the Foraminifera as belonging to different 

 generations. The megalospheric form is produced by an asexual 

 process — the protoplasm of the microspheric parent emerging 

 from the shell and breaking up into a number of spheres, the 

 megalospheres, the initial chambers of the megalospheric form. 



The precise origin of the microspheric form is not yet clearly 

 made out, but there is some evidence in support of the view that 

 it results from the conjugation of the zoospores produced by 

 the megalospheric form, that is, that it is produced by a sexual 

 process. 



The relative sizes of the zoospore and microsphere, and the 

 scarcity of the microspheric form as compared with the megalo- 

 spheric are in favour of this view and Schaudinn's observation of 



