82 SURVEY OF THE REPRODUCTIVE PROCESS 



Huxley, and undoubtedly from a very natural assemblage, 

 for in essentials a very complete unity of organization is to 

 be traced through all the species. Along with this, how- 

 ever, there is so much variety in many adventitious points, 

 affecting both the development of particular organs, and 

 their mode of arrangement, that the general appearance is 

 often strikingly unlike in the different sections of the group. 

 The reproductive process seems to present a corresponding 

 diversity, some species furnishing the most remarkable 

 examples of alternation, while in others no phenomena of 

 the kind have yet been noted. How far these variations 

 may be brought within the scope of a common law will 

 afterwards be considered ; at present it may be sufficient to 

 remark that in the protomorphic or early development of 

 the Ccelenterata, the principal point of interest is that the 

 germinal mass becomes covered with a membrane bearing 

 cilia, by whose play it moves freely through the water like 

 an infusory animalcule. When the action of the cilia ceases, 

 a mouth is formed on one side, and the embryo, assuming a 

 cup-shaped form, becomes transformed into a polype. 



Gemmation is generally a very conspicuous feature of the 

 polypiform phase. It is by repeated pullulation of gemmse, 

 and their continued adhesion to the parent stock, that those 

 composite structures are formed, so characteristic of the 

 group, which have received the name of zoophytes or plant- 

 animals, from their resemblance to ramose growths of a 

 vegetable nature. 



The polypiform condition is the most permanent stage of 

 development, and for this and other reasons, which will 

 afterwards be given, it is here considered as the orthomor- 

 phic or typical phase in the genetic cycle or life history of 

 the species. In many cases, however, it is not that which 

 matures the sexual organs. In the higher division, indeed, 

 of the group (Actinozoa of Huxley) containing the Asteroid 

 and Helianthoid polypes, and also in the common Hydra, 



