132 GENERAL PRINCIPLES OF ZOOLOGY 



and organization. Between polyp and medusa the difference is so great 

 that for a long time these two, though stages of the same species, were 

 referred to different classes of the animal kingdom. In many cases the 

 alternation of generations may be still further complicated by two asexual 

 generations following each other, before the return to the sexual genera- 

 tion takes place. 



Heterogony is distinguished from metagenesis by the fact that the 

 asexual generation is replaced by parthenogenesis. Consequently there 

 alternate animals of sometimes quite different structure, one arising from 

 fertilized, the other from unfertilized, eggs. Certain Crustacea, the 

 Daphnidoe, show heterogony 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 parthenogenetic 

 generations arise. Very often heterogony has been insufficiently distin- 

 guished from metagenesis, parthenogenetic reproduction being regarded 

 as an asexual mode, as was the case in the trematodes. The sexually 

 ripe Distomum produces very peculiar sporocysts; these again give rise 

 parthenogenetically to the larvae of Distomum, the cercaria?. For a long 

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

 arose were not eggs, but 'internal buds'. On the other hand there have 

 been included under heterogony modes of reproduction in which no 

 parthenogenesis whatever occurs, but in which only different forms and 

 organization alternate. A hermaphroditic worm, formerly called 

 Ascaris nigrovenosa, lives in the frog's lungs; it produces the separate- 

 sexed Rhabdonema 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. They are: (i) the maturation of the egg; (2) the process of 

 fertilization; (3) the process of cleavage; (4) the formation of the germ- 

 layers. 



i. Maturation. 



The egg (oocyte) with the large vesicular nucleus cannot yet be fertilized ; 

 it must undergo a series of changes the process of maturation, which 

 consists in the replacement of the germinal vesicle by a much smaller 

 egg-nucleus, and the formation at one pole of the egg of the 'directive 

 corpuscles' or 'polar bodies ' 



