132 GENERAL TRIXCIPLES OE ZOOLOGY 



and ori:;anization. Between polvp and medusa the difference is so great 

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

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

 alternation of generations may be still further complieateii l)y two asexual 

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

 tion takes place. 



Heterogony is distinguished from metagenesis by the fact that the 

 asexual generation is replaceil liv parthenogenesis. Conser[uently there 

 alternate animals of sometimes quite dilTerent structure, one arising from 

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

 Daphnida\ 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,' wliich now are formed, from which again piarlhenogenetic 

 generations arise, ^'ery often heterogony has been insulTiciently distin- 

 guished from metagenesis, parthenogenetic reproduction being regarded 

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

 ripe DistomuDi produces very peculiar sporocysls; tliese again give rise 

 parthenogenetically to the larva: of Disiomum, the cercaria?. For a long 

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

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

 been included under heterogony motles of reproduction in wliich no 

 parthenogenesis whatever occurs, but in which only different forms and 

 organization alternate. A hermaphroditic worm, formerly called 

 Ascaris nigrovcnosa, lives in the frog's lungs; it produces the separale- 

 sexed Rlmlnhviaiia nii^rovoiosiiiii li\ing in mud, from whose eggs the 

 ascarid of the frog is again produced. 



GENER.-\L PHENOMENA OF SEXUAL EEPROPUCTION. 



In sexual reproduction a series of developmental processes is observed 

 vdiich is repeated in an essentially similar manner in all multicellular 

 animals. They are: (i) the maturation of the egg; (j) the process of 

 fertilization; (3) the process of ciea\-age; (4) the formation of the gerni- 

 layers. 



I. Maltiralioii. 



The egg (oocyic) with the large vesicular nucleus cainiot yet lie 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 

 cgg-iiiuiciis, and the formation at one pole of tlie egg of the 'directive 

 corpuscles' or 'polar jjodies ' 



