lOi 



COMPARATIVE ANATOMY 



CHAP. 



heterogeneous elements of independent origin, connective tissue, muscu- 

 lature; nervous system, and sexual organs. The outer epithelium gives 

 rise principally to the connective tissue and the nervous system, while 

 the musculature and the sexual organs may be produced either by the 

 outer or the inner epithelium. 



It is evident that the development of such an intermediate layer, 

 which we call mesoderm, is the necessary preliminary of a higher 

 organological differentiation of the body. 



XII. Reproduetion. 



Asexual reproduction by fission and gemmation — Stock formation — Division 

 of labour and polymorphism. 



Asexual reproduction is very common among the Cnidaria side by 

 side with sexual reproduction. Among the Ctenophwa alone it has not 

 been observed. In Hydra we find asexual reproduction by gemmation 



Fig. 78.— Bougainvillea ramosa (after Allman), with 'bndding Medusa;. A, Nutritive polyps ; 

 mfc, medusa buds ; m, detaclied young MedusEe (Margells ramosa). 



side by side with sexual reproduction in adult animals. Buds are formed 

 by hollow outgrowths of the body wall. These buds grow, and at the 

 distal end a breach is formed — the oral aperture, round which the 

 tentacles arise by means of new outgrowths. Such buds can detach 



