236 STRUCTURE AND LIFE-HISTORY OF VOLVOX GLOBATOR. 



life-history would be incomplete without a process of sexual reproduc- 

 tion, and accordingly, after a long sequence of asexual generations, a 

 strictly sexual process intervenes, from which result certain spores 

 destined to lie dormant for awhile, and, like the zygospores of the 

 Conjugate Algse, to resist vicissitudes of condition and climate through 

 the rigours of winter, and then to reproduce the parent-form in the 

 succeeding year, when external conditions again favour its development. 



Cohn first fully traced the various stages of this process, and described 

 them in the " Beitrage zur Biologie der Pflanzen," 1875, Vol. I., Heft 3, 

 and in the " Annales des Sciences Naturelles," 4 ieme Ser. Bot ; Tom. 

 V. 323 ; and his observations have been more or less confirmed by other 

 investigators, especially by Carter, Ann. Nat. His., 3rd Ser., Vol. LTI., 

 1859, p. 1, and more recently, in 1877, by a French botanist, M. F. 

 Henneguay. 



Cohn and Carter both hold that there are two varieties of Volvox, 

 one monoecious, the other dioecious, and the latter maintains that 

 Sphaerosira Volvox is the male form of the dioecious sub-species. Be 

 this as it may, the reproductive process in the monoecious form is as 

 follows : — The sexual reproductive cells, male and female, occur in 

 spheres of unusual size in the autumn, and are few in proportion to the 

 number of sterile cells, and the reproductive process does not occur 

 simultaneously with, but as a climax to a long series of asexual generations. 

 On their first appearance the gynogonidia or female cells are about three 

 times the size of the sterile ones, of a deep green colour, and of a frothy 

 consistency from abundance of vacuoles ; they are easily distinguished 

 from the parthenogonidia by their never subdividing. (Plate "VJLLL, 

 Fig. 16.) They next become flask-shaped, their narrow end touching 

 the periphery of the sphere, and the broader end hanging free in the 

 internal cavity. (Plate VHI., Fig. 1, 62.) Finally they assume a 

 spherical form, and become oo-spheres, each enveloped in a gelatinous 

 membrane. (Plate Vm., Fig. 1, 63, 64.) 



The androgonidia or male cells at first closely resemble the partheno- 

 gonidia, but, undergoing division in two instead of three directions, 

 develop into plates or discs of cells, not into spheres, and ultimately resolve 

 themselves intp bundles of naked elongated cells, in which the chloro- 

 phyll is transformed into a reddish pigment, each with a long colourless 

 beak, with a red " eye-spot " and two cilia. (Plate VLTI, Fig. 1, a, a2.) 

 About the same time that the oo-sphere is mature these antheridia begin 

 to move from the combined action of their cilia, (Plate "VTH., Fig. 2,) and 

 then break up into separate antherozoids, which finally become free and 

 move rapidly within the cavity of the sphere. (Plate VLTI., Fig. 1, aS.) 

 Assembling round the oo-spheres, they penetrate the envelopes of the 

 latter, (Plate VLTI., Fig. 3,) coalesce with their contents, and the 

 oo-sphere thus fertilised becomes an oospore, which soon develops a cell- 

 wall covered with conical stellate'projections, and a second smooth internal 

 membrane. (Plate VIII. , Fig. 4.) The chlorophyll now gradually 

 disappears, and is replaced by an orange red pigment. In this condition 

 the oo-spore constitutes the Volvox stellatus of Ehrenberg. It is liberated 



