1 6 EMBRYOLOGY. 



explanation is here practically the same as in the case of Aphis, 

 and is paralleled in the gemmiparous series by the production of 

 buds in the larval forms of Trematodes, etc. A very similar 

 occurrence takes place in Ascaris nigrovenosa (vide chapter on 

 Nematoidea), except that larval forms, which carry on reproduc- 

 tion and then perish without developing farther, do so by a true 

 sexual process. Thus there is an alternation of generations of 

 adult and larval sexual forms. The Axolotl is an intermittent 

 example of the same phenomenon. 



As might be anticipated from the mode in which alternations 

 of generations have become established, incomplete approxi- 

 mations to it are not uncommon. Such approximations are 

 especially found in the Arthropoda, where alternations of sexual 

 and parthenogenetic generations frequently take place, in which 

 the individuals of different generations are similarly organised 

 (Psychidae, Apus, &c.). Another approximation is afforded by 

 the parthenogenetic winter eggs of Leptodora amongst the 

 Phyllopods, which give rise to Nauplius larvae, while the young 

 hatched from the summer eggs do not pass through a meta- 

 morphosis. Numerous transitional cases are also found amongst 

 the forms in which there is an alternation of sexual and gemmi- 

 parous generations. 



The whole of the cases to which allusion has been made in 

 this section may be conveniently classed under the term alterna- 

 tions of generations, but the cases of alternation of two sexual 

 generations, and of sexual and parthenogenetic generations, 

 are classified by Leuckart, Claus, etc. as cases of heterogeny, 

 which they oppose to the other form of alternation of genera- 

 tions. If special terms are to be adopted for the two kinds of 

 alternation of generations, it would be perhaps convenient to 

 classify the cases of alternations of sexual and gemmiparous 

 generations under the term metagenesis, and to employ the 

 term heterogamy for the cases of alternation of sexual and 

 parthenogenetic generations. 



The term Nurse (German Amme), employed for the asexual 

 generations in metagenesis, may advantageously be dropped 

 altogether. 



