532 PROFESSOR W. C WILLIAMSON ON VOLVOX GLOBATOK. 



There is evidently an intercellular substance of some kind, 

 in addition to the outer common investing membrane, 

 which supports the filaments externally, and encloses the 

 cells within it. The investing membrane is most probably 

 the result of an alteration and condensation of the outer 

 walls of some of the primary cells of the young germ. 

 Since all the existing cells were originally developed within 

 others, it is evident that the walls of the latter have either 

 been absorbed, or they still exist in the form of thin layers, 

 investing the cells to which they gave birth. The latter 

 is the more probable conclusion of the two ; by their de- 

 velopment and consolidation they may have produced both 

 the intercellular substance and the common investing in- 

 tegument. The existence of the former of these tissues 

 appears to be established by specimens that have been acted 

 upon in the way represented by Fig. 11. 



When these objects have been mounted a few days in a 

 solution of iodine, which, by the way, renders their cilias 

 beautifully distinct, the cell-contents separate into two dis- 

 tinct portions. One of these is homogeneous, and of a 

 pale green colour ; the other, which comprehends the lining 

 membrane and the granular substances, assumes a dark- 

 brown hue. 



The existence of a true internal cell or membrane lining 

 each cell, distinct from the outer cell-wall, is obvious from an 

 examination of young half-developed individuals, in which 

 the cells are of comparatively large size. On subjecting 

 one of these to gentle pressure under water, so as slightly 

 to rupture it, the green cell-contents soon flow out, as al- 

 ready described. As they do so, we see that their form is 

 capable of modification, enabling them to glide through a 

 very narrow fissure, or to be packed together in a small 

 space ; but each one retains its pristine integrity ; and, 

 as soon as the pressure is removed, resumes its spherical 

 .form. If, on the other hand, the pressure is increased. 



