GENERAL EMBRYOLOGY. 145 



that, in alternation of generations, there is not only a difference in 

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

 form and organization. Between polyp and medusa the 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 many cases the alternation of generations 

 may 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 by the fact that 

 the asexual generation is replaced by parthenogenesis. Conse- 

 quently there alternate animals of sometimes quite different struc- 

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

 unfertilized, eggs. Certain Crustacea, the JJaphnidae, 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 appear for a short time; they fertilize the 

 ' winter eggs/ which now are formed, from which again partheno- 

 genetic generations arise. Very often heterogony has been insuffi- 

 ciently distinguished from metagenesis, chiefly for the reason that 

 parthenogenetic reproduction was regarded as an asexual mode, as 

 was the case in the trematodes. The sexually ripe Distomum 

 produces very peculiar sporocysts; these again give rise partheno- 

 genetically to the larvae of Distomum, the cercariae. For a long 

 time the erroneous view was held that the cells from which the 

 cercariaa arose were not eggs, but l internal buds/ 'germinal 

 granules/ On the other hand there have been included under 

 heterogony modes of reproduction in which no parthenogenesis 

 whatever occurs. Cases have been called heterogony when two 

 generations which have only different forms and organization 

 alternate. Ascaris nigrovenosa, an hermaphroditic worm, lives in 

 the frog's lungs; it produces the separate-sexed Rliabdonema 

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

 frog is again produced. 



GENERAL PHENOMENA OF SEXUAL REPRODUCTION. 



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. They are: (1) the maturation of the egg; (2) the 

 process of fertilization; (3) the process of cleavage; (4) the forma- 

 tion of the three germ-layers. 



