214 SUMMARY OF CONCLUSIONS. 



organs, after the typical form of the species has been 

 acquired (as in the Polypifera and Ferns.) 



IX. In the orthomorphic or intermediate period, the 

 gemmae generally remain in adhesion, to form a compound 

 organism (as in zoophyte and most plants), but a few cases 

 are known in which they give rise to a well-marked alter- 

 nation of distinct forms (as in the Aphides.) 



X. It is but rarely that alternation occurs in more than 

 one stage of the development of the same species. Even 

 in the larger groups or orders characterized by such pheno- 

 mena, their marked occurrence is mostly confined to one of 

 the life periods above specified. Among the Cestoid worms, 

 however, we find distinct alternation, both in the protomor- 

 phic and gamomorphic stages ; and in the Polyzoa, and 

 some Annelida, less obvious processes of the same kind 

 appear to occur in all the stages. 



XL The gemmation, which occurs in any stage, may oc- 

 casionally be lengthened out by the successive pullulation of 

 a series of zooids or phytoids of the same general character, 

 previous to the supervention of the characteristic pheno- 

 mena of the next succeeding stage. The number of inter- 

 polated links is fixed in some species, and variable in others. 

 Such pullulation may occur at any stage, but is most com- 

 mon in the orthomorphic, giving rise in it to the compound 

 ramified structures characteristic of plants and zoophytes. 



XII. In the embryogeny of the higher species, a certain 

 parallel may be traced to the first or protomorphic form of 

 alternation, in the implantation of the primitive trace of the 

 typical organization, on the cellular mass into which the 

 contents of the ovum are resolved, by the process of cleav- 

 age following immediately on impregnation. This view is 

 founded on the following considerations : 



1. The duplication, in whole or part, of the embryonic 

 axis, as an occasional abnormality, even in the higher 

 species, resulting in the formation of a double monster. 



