CLASSIFICATION OF THE PRIMORDIA 83 



state of equilibrium of such a kind as to suppress any cause 

 which might bring about a new division. ^ In a specimen a 

 consisting of 2*= i6 cells, the same state has been reached after 

 the fourth division. Therefore it may be said that a is younger 

 than b in respect of the property under consideration. (This 

 remark is also appUcable on Spirogyra, § 59.) 



As soon as the cells are connected with one another the sensi- 

 tive period of the property number is over. Since the cells have 

 not yet reached their definitive dimensions, the sensitive period 

 of the primordium dimensions [size) begins about that moment. 

 The sensitive periods of number and size being independent, 

 both properties are also independent with regard to their 

 plasticity. (Compare Spirogyra, § 59.) An adult specimen with 

 small cells is younger than a specimen with large cells in respect 

 of the property size, whatever may be the number of cells. A 

 given difference between two adult specimens which belong to 

 the same species depends on plasticity ; the same difference 

 between two specimens which belong to distinct species de- 

 pends on a difference in their living mixture if the conditions 

 of existence have been the same through the whole duration 

 of their development. 



§ 67.— VARIATION STEPS IN PEDIASTRUM.— See on 

 VARIATION STEPS, Part VII. 



§68.— SPECIAL SYSTEMS OF SEGMENTATION {con- 

 tinued). IV. VOLVOX, HYDRODICTYON.— In these Algce 

 the cells are imited into a very characteristic system of equi- 

 librium (society),^ which is, as far as I know, distinctly different 

 from any other system in the animal and vegetable kingdoms. 



As in Pediastrum, the initial cell undergoes a certain number 

 of successive divisions, the produced cells separating after each 

 division. When the definitive number is reached, all the hitherto 

 independent cells unite at once into a coenobium, which is an 

 elegant hollow sphere or utricle. There is thus no morpho- 

 logical relation between the relative position of the segments 

 (cells) in an adult specimen and the preceding states of develop- 

 ment. (Compare Pediastrum, § 64, and the Myxomycetes, § 65.) 



§ 69 {continued from § 61, p. 77).— SYSTEMS OF SEG- 

 MENTATION DERIVED FROM THE UNIAXIAL SYSTEM. 

 RAMIFICATION AND CLEAVAGE.— We have seen, in § 61, 

 that the uniaxial system is a primary system, from which 

 several other systems are derired. 



iThe equilibrium alluded to is, of course, distinct from the equilibrium 

 which exists between the cells when they have met one another. 

 * See G. S. WEST, loc. cit., pp. 195, 207. 



