174 GENERAL PRINCIPLES OF ZOOLOGY, 



before the return to the sexual generation takes place ; one 

 speaks then of grand-nurse, nurse, and sexual animal. 



Heterogony is distinguished from the ordinary alterna- 

 tion of generations, or metagenesis, by the fact that the 

 asexual generation is replaced by parthenogenesis. Con- 

 sequently there alternate animals of sometimes quite dif- 

 ferent structure, of which the one arises from fertilized, 

 the other from unfertilized, eggs. Certain crabs, the DapJi- 

 nidcc, show heterogony in a typical manner. During a 

 long period of the year, only females are found ; these in- 

 crease parthenogenetically by " summer eggs" ; then males 

 appear for a short time; they fertilize the " winter eggs," 

 which are meantime produced, from which again partheno- 

 genetic generations arise. Very often heterogony has been 

 insufficiently discriminated from metagenesis, chiefly for 

 the reason that parthenogenetic reproduction was regarded 

 as an asexual mode, as was the case in the Trcuiatodes. 

 The sexually ripe Distoma produces the very abnor- 

 mally formed sporocysts ; these again give rise partheno- 

 genetically to the larvae of the Distoina, the Ccrcarice. 

 For a long time the erroneous view was held that the 

 cells from which the Cercariae arose were not eggs, but 

 " internal buds," " germinal granules." On the other hand 

 there have been included under heterogony modes of repro- 

 duction in which no parthenogenesis whatever occurs. 

 Cases have been called heterogony, when two generations 

 which have only different forms and organization alternate. 

 In the frog's lungs, lives Ascaris nigrovenosa, an herma- 

 phroditic worm ; it produces the separate-sexed RJiabdoncina 

 nigrovenosmn living in mud, from whose eggs the ascarid of 

 the frog is again produced. 



Asexual and Sexual Reproduction. The distribu- 

 tion of the modes of reproduction in the animal kingdom 

 shows, in an almost convincing manner, that asexual is the 

 lowly, sexual the highly, developed type of reproduction. 

 Asexual reproduction predominates among the Protozoa; 

 among the ctrlcntcratcs, it is found together with the 

 sexual method ; it disappears in case of the worms, and. 



