PRESIDENTIAL ADDBESS. 589 



If a large batch of iudividuals of Polystomella crispa be killed with a reagent 

 which dissolves the shell, though preserving its protoplasmic contents, it will be 

 found, on examining the casts so obtained, that besides those of the type described 

 and figured by Williamson with a comparatively large initial chamber (about 60 /x), 

 and these are by far the most abundant, there are others in which the initial 

 chamber is much smaller (about 10 ii). In other words, megalospheric and micro- 

 spheric iudividuals occur in the batch, as among the fossil shells of Nummuhtes, 

 preserved in the Eocene strata. 



On staining them another point of difference appears. A single large nucleus 

 is found in the majority of the megalospheric forms, while in the microspheric a 

 number of small nuclei lie in the chamljers most remote from the mouth of the 

 shell. 



The result of observations on the living and preserved animals may be briefly 

 stated as follows : — 



The Microspheric Form. 



The mici'ospheric form has many small nuclei, even at auoaiiy stage of growth. 

 These nuclei consist of a homogeneous ground substance with many small nucleoli 

 scattered through it. They lie in the chambers near the centre of the shell, and 

 increase in number by simple division. They also exhibit a remarkable pheno- 

 menon to which I shall have to recall your attention later. Though several of the 

 nuclei, and especially those that have recently divided, have a rounded contour, 

 many of them are highly irregular in outline, giving oft" processes which extend in 

 branching irregular strands, staining deeply with nuclear sta,ins, into the protoplasm. 

 Free shreds of such strands lie scattered in the chambers in the neighbourhood of 

 the nuclei, and in large specimens of the microspheric form it is common to find 

 the protoplasm crowded with such deeply staining strands, and with no trace 

 to be found of the rounded nuclei present in the earlier stages. It is difficult to 

 avoid the conclusion that the nuclei, after increasing in number by amitotic 

 division, give off" the strands and are ultimately wholly resolved into them. 



In a culture of Polystomella 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. In the early 

 phases these specimens are distinguished by a great increase in the number of 

 pseudopodia issuing from the shell, so that the latter appears 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 supporting surface and within the area 

 formerly covered by the halo. The internal protoplasm 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 separates into as many spherical masses, 

 which remain connected by a felt of hyaline pseudopodia. Some 200 is a common 

 number 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 been formed, a small 

 aperture being left in each for the passage of the pseudopodia. After lying in close 

 contact for some hours, the spheres rapidly and simultaneously draw apart from 

 one another, and within half an hour from the beginning of the movement they are 

 dispersed over a wide area, and each becomes the centre of a system of a pseudo- 

 podia of its own. 



The whole of the protoplasm of the parent is used up in the formation of the 

 brood of young, the shell being left empty. The process from the first appearance 

 of the halo to the dispersal of the young is complete in about twelve hours. 



In a short time the protoplasm which lies outside the aperture of each of the 

 spheres secretes the wall of a second chamber of characteristic shape, and the young 

 individual is then clearly recognisable in size and shape as the two-chambered 

 young of the megalospheric form. Each of the spheres was, in fact, a megalo- 

 sphere. The microspheric parent has given rise to, indeed it has become, a brood 

 of megalospheric young. 



