Williamson on Volvox glohator. 
53 
tions of the cells being those which were below the points of 
mutual contact. 
The direction taken by these threads frequently demonstrates 
the presence of invisible cell-walls midway between two pro- 
toplasms. We have already seen that when the threads are 
liberated their tendency is to become shortened ; hence they 
pursue the most direct course from one point to another. But 
we occasionally see examples of what is delineated in fig. 10 e 
where two threads run parallel for some distance and then 
suddenly diverge, proceeding to different protoplasms. This 
condition obviously indicates the existence of some invisible 
point d^appui, where the divarication occurs. This, doubt- 
less, consists of the hyaline cell-wall. We obtain similar evi- 
dence from almost every example of Volvox, when we examine 
the protoplasms at one margin of the sphere in profile. This 
can readily be done with object-glasses of short focus, owing 
to the transparency of the tissues. We very frequently see 
that the threads, instead of being straight, dip inwards towards 
the centre of the sphere, and meet at a well-defined angle mid- 
way between the two protoplasms, as represented in fig. 12 e. 
This can only be explained in the way just suggested. 
Whatever may be the true nature of the objects which I re- 
gard as cells, they are obviously separated from the protoplasms 
before the latter assume their stellate forms, or develop their 
delicate connecting threads. I have had specimens of every 
age, from the gemma artificially liberated from the parent 
sphere, to the hyaline and matured individual, in all of which 
these cells exist. They are obviously developed immediately 
after the final process of segmentation has completed the re- 
quired number of protoplasms ; the development of these cells 
of the cilia and of the common pellicle apparently taking place 
about the same time. Names sometimes matter little ; but 
here they are significant, since they involve the origin of the 
disputed structure. Mr. Busk regards it as the outer covering 
of the protoplasm dropsically distended ; I believe that the 
outer covering (my protoplasmic membrane) remains in close 
connexion with the viscous protoplasm until the two are sepa- 
rated artificially, and that the cell is a secretion from the outer 
surface of the protoplasmic membrane. According to this ex- 
planation, the two bear the same mutual relations as exist 
between the outer and inner membranes of any other Confervoid 
cell. The beautiful stellate, spiral, and other forms which the 
inner membranes of many of these plants assume after their 
separation from the cell-wall, with wliich they were primarily 
in close contact, afford ready illustrations of the similar trans- 
formations in Volvox. 
