254 DEVELOPMENT OF VOLVOX GLOBATOR. 



aggregation originated. In the midst of the polygonal masses of 

 endochrome, one mass (a), rather larger than the rest, is seen to 

 present a circular form : and this, as will presently appear, is the 

 originating cell of what is hereafter to become a new sphere. The 

 growing Volvox at first increases in size, not only by the interposi- 

 tion of new hyaline substance between its component masses of 

 endochrome, but also by an increase in these masses themselves 

 (Fig. 2, a), which come into continuous connexion with each other 

 by the coalescence of processes (b) which they severally put-forth ; 

 at the same time an increase is observed in the size of the globular 

 cell (c), which is preliminary to its binary subdivision. A more 

 advanced stage of the same developmental process is seen in Fig. 3 ; 

 in which the connecting processes (a, a) are so much increased in 

 size as to establish a most intimate union between the masses of en- 

 dochrome, although the increase of the intervening hyaline substance 

 carries these masses apart from one another ; whilst the endo- 

 chrome of the central globular cell has undergone segmentation into 

 two halves. In the stage represented in Fig. 4, the masses of en- 

 dochrome have been still more widely separated by the interposition 

 of hyaline substance ; each has become furnished with its pair of 

 ciliary filaments ; and the globular cell has undergone a second seg- 

 mentation. Finally, in Fig. 5, which represents a portion of the 

 spherical wall of a mature Volvox, the endochrome-masses are ob- 

 served to present a more scattered aspect, partly on account of their 

 own reduction in size, and partly through the interposition of a 

 greatly-increased amount of hyaline substance which is secreted from 

 the surface of each mass ; and that portion which belongs to each 

 cell, standing to the endochrome-mass in the relation of the cellulose 

 coat of ordinary cells to their primordial utricle, is frequently seen 

 to be marked-out from the rest by delicate lines of hexagonal areola- 

 tion (c, c), which indicate the boundaries of each. Of these it is 

 often difficult to obtain a sight, a nice management of the light 

 being usually requisite with fresh specimens ; but the prolonged 

 action of water (especially when it contains a trace of iodine), or of 

 glycerine, will often bring them into clear view. The prolonged 

 action of glycerine, moreover, will often show that the boundary 

 lines are double, being formed by the coalescence of two contiguous 

 cell-walls ; and they sometimes retreat from each other so far that the 

 hexagonal areola? become rounded. As the primary sphere ap- 

 proaches maturity, the large secondary germ-mass, or Macro-goni- 

 dium, whose origin has been traced from the beginning, also advances 

 in development ; its contents undergoing multiplication by suc- 

 cessive segmentations, so that we find it to consist of 8, 16, 32, 64, 

 and still more numerous divisions, as shown in Figs. 6, 7, 8. Up 

 to this stage, at which first the sphere appears to become hollow, 

 it is retained within the hyaline envelope of the cell within which 

 it has been produced ; a similar envelope can be easily distinguished, 

 as shown in Fig. 10, just when the segmentation has been completed, 

 and at that stage the cilia pass into it, but do not extend beyond 



