l8o THE GERM-PLASM 



the germ-plasm is composed in equal numbers, the periods of 

 activity of which alternate with one another. The ids of the 

 accessory germ -plasm, which arose subsequently, must be larger 

 than those of the ancestral germ-plasm, because they contain 

 more numerous determinants. If at some future time it should 

 be definitely ascertained that those granules or microsomes, 

 which are arranged like beads in a necklace in the nuclear rods, 

 really correspond to ids, we may possibly, perhaps, be able to 

 prove by the aid of the microscope that such differences in size 

 actually exist. A knowledge of the entire number of nuclear 

 rods or idants may also possibly help to confirm the theory, 

 for it is probable that in species in which alternation of genera- 

 tions occurs, the ids, and therefore the idants also, have been 

 doubled during the development of the species. For if my view 

 is correct that a definite amount of germ-plasm is necessary for 

 the normal development of a certain kind of egg, the periodical 

 inactivity of half the ids must have been accompanied by a cor- 

 responding doubling of these structures. 



The mechanism of the idioplasm in alternation of generations 

 becomes somewhat different, and rather more complicated, as 

 soon as the second generation arises, not from a single cell, but 

 from several cells at the same time, derived from different layers 

 of the body. This is the case as regards t/ie strobilatio7i of the 

 higher inedusce and that of tape-'wor?ns, and an intermediate 

 stage is seen in \\\e gemination of the SalpcF. 



In the last-named animals, two generations differing as regards 

 the form of the body and mode of reproduction follow one 

 another. A number of individuals are united into a ' chain ' in 

 the first generation, in which sexual multiplication occurs ; and 

 in the second generation the individuals are separate, and mul- 

 tiply by budding. It has already been pointed out in the chapter 

 on gemmation that this budding is produced by a co-operation 

 of the ectoderm and mesoderm cells. We must imagine that in 

 this case, again, the germ-plasm of the egg- and sperm-cells is 

 composed of two kinds of ids, which alternately become active, 

 one of which contains the determinants for gemmation, and the 

 other those for embryogeny. In the case of the Hydroid-polypes 

 and medusae, the determinants of the ' blastogenic ' ids remdn 

 together in ofie cell, but in the single form of Salpa they must 

 be separated into groups during embryogeny, and these groups 

 would be supplied — in part to the ectoderm, and in part to the 



