GENERAL SJIBRYOZOGT. 145 



tliat, iu alternation of generations, there is not only a difference in 

 tlie mode of reproduction, but usually, in addition, a difference in 

 form and organization. Between polyp and medusa tlie difference 

 is so great that for a long time these two, though representatives 

 of the same species, were referred to quite different classes of the 

 animal kingdom. In nian.y cases tlie alternation of generations 

 nniy be still further complicated by two asexual generations fol- 

 lowing each other, before the return to the sexual generation takes 

 place; one speaks then of grand-nurse, nurse, and sexual animal. 



Heterogony is distinguished from metagenesis liy the fact that 

 the asexual generation is replaccil by parthenogenesis. Conse- 

 cpiently there alternate animals of sometimes quite diff'erent struo 

 ture, of which the one arises from fertilized, the otlier from 

 imfertilized, eggs. Certain Crustacea, the Daphnidai, show heter- 

 ogony in a typical manner. During a large part of the year only 

 females are found; these increase parthenogenetically by ' summer 

 eggs'; then males ajipear for a short time; tliey fertilize the 

 'winter eggs,' wliich now are formed, from wdiich agaiii partlieno- 

 genetic generations arise. Ver}' often heterogony has been insuffi- 

 ciently distinguished from metagenesis, cliiefly for the reason that 

 parthenogenetic reproduction was regarded as an asexual mode, as 

 was the case in the trematodes. Tlie sexually ripe Distomum 

 produces very peculiar siiorocysfx; these again give rise partheno- 

 genetically to the larvai of Disioinum, the cercariw. For a long 

 time tlie erroneous view was held that the cells from wliich the 

 cercariffi arose were not eggs, but 'internal liuds,' 'germinal 

 granules.' On the other Inind there have been included under 

 heterogony modes of reproduction in which no purtlienogenesis 

 whatever occurs. Cases have been called heterogony wlien two 

 generations which have only different forms and organization 

 alternate. Ascaris iii[/r(iri.'jio.ia, an liermapliroditii; worm, lives in 

 the frog's lungs; it produces tlie separate-sexed Jihabdoneina 

 nigrovenosiim living in mud, from whose eggs the ascarid of the 

 frog is again produced. 



(JEXERAL PIIEXOMEXA OF SEXUAL KEI'ItOIU'i TIOX. 



In sexual reproduction a series of developmental processes is 

 observed which is repeated in an essentially similar manner in all 

 multicellular animals, and hence these should be spoken of here 

 together. Tliey are: (1) the maturation of tlie egg; (2) the 

 process of fertilization; (.'j) tlie process of cleavage; (4) the forma- 

 tion of the three frerm-lavers. 



