44 STRUCTURE AND LIFE-HISTORY OF VOLVOX GLOBATOR. 



application of tlie re -agent, and then only where the hexagonal structure 

 exists,) I have no doubt that this inner layer is the true " primordial 

 utricle " of tlie cell, and possesses that character of vital and formative 

 matter which distinguishes this element of cell-structure from the outer 

 wall, which, on the other hand, probably consists of cellulose or some 

 similar compound. Probably the arrangement of this inner layer in 

 radiating lines or ribs contributes to the elasticity of the fabric, whereby 

 it is enabled to open at a given point for the escape of the young, and to 

 contract again after their emission. 



The increase of individuals by the means already described is strictly 

 an instance of subdivision. 



But Volvox Globator also affords an instance of true alternation of 

 generations. As may probably be affirmed of all living organisms, its 

 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 Algae, 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 " Beitriige zur Biologic der Pflauzen," 187o, Vol. I., Heft 3, 

 and in the " Annales des Sciences Naturelles," 4 i^me 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. III., 

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

 Henueguay. 



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

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

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

 this as it may, the reproductive process in the monoscious 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 gyuogonidia 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 VIII., 

 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 VIII., Fig. 1, 62.) Finally they assume a 

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

 membrane. (Plate VIII., 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, 



