OTHER rORMS OF ALTERNATION. KIT 



any indications of gemmation, in the orthomorpliic stap,e, 

 as distinct from growth, are very equivocal ; possibly, how- 

 ever, we may trace them in such phenomena as the renewal 

 of the hair, teeth, antlers, and other cutaneous appendafi;es, 

 or of the whole integument in certain species — the limited 

 reproduction of lost parts — the abnormal sprouting of su- 

 pernumerary limbs — and the still rarer monstrosity termed 

 foetus in foBtu. Perhaps, too, some of the normal struc- 

 tures of foetal life may be considered as early orthomorpliic 

 or larval gemmae ; e.g., the placenta of Mammalia, the 

 allantois, the branchial tufts of Batrachia, and the ciliated 

 lobes of Gasteropoda. 



§ 2. We come next to enquire into the phenomena 

 which, in the higher animals, may be regarded as standing 

 in the place of the variety of alternation depending on gem- 

 mation in the gamomorphic stage. To this a certain 

 analogy has just been indicated in the processes connected 

 with the development of the reproductive elements, which 

 may be held to represent this modification of alternation, 

 somewhat in the same way that the succession of forms in 

 embryogeny shadow^ out the budding off of zooids in the 

 rudimentary or protomorphic stage. 



Some of the points which suggest such a correspondence 

 will now be briefly noticed. 



§ o. To begin Avith one of minor importance, it may be 

 observed that the periodicity whicli so commonly charac- 

 terizes the development of the sexual organs seems to as- 

 sert for them, even in the higher species, a relation to the 

 body at large somewhat different from that hold by the 

 other members. Much in the same way, for instance, that 

 the " Hydra Tuba," after a certain lapse of polype life, 

 matures a succession of reproductive structures, in the guise 

 of medusa-buds, and on tlieir detachment relapses again 

 into the ordinary polype life, till other periods of reproduc- 

 tive activity occur, marked by similar phenomena ; so we 



