102 



PROFESSOR ALLMAN ON THE GENETIC SUCCESSION 



proper limits, when he denned it as " the total result of the development of a 

 single ovum," and compared the definite groups of zooids which constitute the 

 life series of animals presenting the phenomenon of " alternation of genera- 

 tions " to the single organisms known as the individuals, which make up the 

 species in other animals. These groups form the periods of the series ; the 

 period repeats itself by true generation, and this repetition continues itself 

 indefinitely, like a circulating decimal, so as to represent the indefinitely 

 extended life of the species, while the life of the individual — in its technical 

 sense as the component of the species — is expressed by each period singly. 



Fig. 4. — Diagram of Laomedea. 

 a ii a a, hydranths belonging to the primary or direct line of succession ; a'a'a'a', hydranths belonging 

 to a secondary or diverging line of succession ; b, blastostyle of the primary line of succession, bearing 

 gonophores, and surrounded by a gonangium ; V, blastostyle with gonophores and gonangium of the 

 diverging line. 



It is a universal law in the succession of zooids, that no retrogression ever 

 takes place in the series. In other words, no bud ever becomes developed into 

 a zooid which is of a different form from the budder, and has at the same time 

 preceded it in the line of succesion. Thus, true hydranths are never emitted 

 either by blastostyle, blastocheme, or gonophore ; and to this law the peculiar 

 gemminate hydriform bodies which are found on the summit of the female 

 blastostyle in certain species of Halecium form no exception ; for though closely 

 resembling true hydranths, they appear to have a different signification, con- 



