ALTERNATION OF GENERATIONS. 117 



sexual organs, represent the typical or orthomorphic form 

 of the species. 



3. That the process of gemination in this case represents 

 a particular step in the course of embryonic development, 

 marking the transition from the early germinal or proto- 

 morphic, to the first beginning of the orthomorphic or 

 typical stage ; and that it has no direct bearing on the evo- 

 lution of the sexual organs — the process which distinguishes 

 the gamomorphic stage. 



The first of these characters belongs to all forms of '' al- 

 temation ;" the latter two are distinctive of that now de- 

 scribed, which may, therefore, be termed " Alternation from 

 gemmation in the Protomorphic stage," or, more shortly, 

 " Protomorphic Alternation." Its distinctness will appear 

 at once, on comparing it with other forms, particularly with 

 that which prevails among the lower or polypiferous section 

 of the Coelenterate division of the Animal .Kingdom (Hy- 

 drozoa of Huxley). 



§ 3. In more than one family of Polypes, the true ova 

 are frequently produced by zooids, varying much in size, 

 but having the general structure of the Medusa?, or common 

 Jelly-fishes of our seas. From the ovum a cup-shaped 

 embryo proceeds, which is transformed into a polype, and 

 from this, as from the original shoot of a plant, there is 

 generally developed, by a series of successive sprouts, a 

 polypidom supporting a whole colony of polypes like the 

 original — variously disposed according to the species, but 

 all organically connected together. At certain seasons, and 

 under favourable circumstances, there are developed either 

 from the polypes or from the polypidom, peculiar bud-like 

 processes, which sometimes acquire the characters of regu- 

 larly organized Medusoids, but at others remain in the ru- 

 dimentary condition of mere spermatic and ovarian sacs. 

 It is fi'om these that the ova are derived, whose course of 

 development has just been noticed. 



