from that of the other by the progressive formation of a 

 double party-wall of integument, attended by progressive 

 separation of one party-wall from the other, and by con- 

 comitant constriction of the body of the Polygastrian, until 

 the vibratile action of the superficial cilia of each separating 

 moiety severs the narrowed neck of union, and they be- 

 come two distinct individuals. An eye-speck, a pulsatile 

 sac, a proboscis, or whatever organ may be required to 

 complete the specific characters of the particular Polygas- 

 trian, are likewise developed, and the individualization of 

 each moiety is thus completed. 



This mode of propagation is termed e spontaneous fission/ 



In the freshwater polype the progeny of the primary 

 impregnated germ-cell retained unaltered in the body, may 

 set up, under favourable stimuli of light, heat and nutri- 

 ment, the same actions as those to which they owed their 

 own origin : certain of the nucleated cells do set up such 

 actions, those, e. g., in the Hydra fusca, which are aggre- 

 gated near the adhering pedicle or foot ; and the result of 

 their increase by assimilation and multiplication is to push 

 out the contiguous integument in the form of a bud, which 

 becomes the seat of the subsequent processes of growth 

 and development : a clear cavity or centre of assimilation 

 is first formed, which soon opens into the stomach of the 

 parent : but the communication is afterwards closed, and 

 the young Hydra is ultimately cast off from the surface of 

 the parent. 



This mode of propagation is termed e Gemmation/ It 

 differs from the development of the Hydra ab ovo inasmuch 

 as the impregnated germ-cell, which set on foot the process, 

 is derivative and included in the body of the adult, instead 

 of being primary and included in a free ovum. But the 

 germ-cell is the essential part of the ovum, and the chorion 

 an accessory and non-essential part. 



The very small size in relation to the entire body, and the 

 superficial position, of the derivative germ-cell which takes 



