94 George Lefevre 



Observations on the annelids were next extended by Fischer 

 ('02, '03) to Amphitrite and Nereis. Mathews ('01) had previously 

 shown that artificial parthenogenesis could be produced in the 

 starfish by mechanical agitation of the eggs, and this Fischer 

 proved to be also true in the case of Amphitrite. He found that 

 the eggs of this worm are extremely susceptible to mechanical 

 shock, and can be brought to the "trochophore stage" by squirting 

 them from a pipette after a residence in sea-water of from one-half 

 to one hour. Fischer, furthermore, found that Ca (N03)2 is capable 

 of inducing parthenogenetic development in Amphitrite, a result 

 which he attributes to the specific effect of calcium ions, although 

 in Nereis he thinks the essential factor is the abstraction of water 

 from the egg caused by the increased osmotic pressure of the sea- 

 water. The morphological phenomena concerned in the develop- 

 ment of the unfertilized eggs of Amphitrite and Nereis are nearly 

 as widely divergent from the normal as in the case of Chaetopterus. 

 The calcium eggs of the former show a totally different appearance 

 from that of eggs fertilized by sperm. Although rarely cleavage 

 may occur in a more or less normal manner as far as eight or 

 twelve cells, the majority of eggs that divide do not go beyond the 

 two-cell stage. Since a larger percentage of eggs reach the swim- 

 ming condition than undergo cleavage, Fischer was inclined to 

 believe that the formation of the trochophore could take place in 

 the absence of segmentation, a result in harmony with Loeb's 

 conclusion for Chaetopterus. In Nereis, parthenogenetic de- 

 velopment is likewise far from normal in character, although 

 cleavage, for the most part very irregular, seems to be of commoner 

 occurrence than in Chaetopterus and Amphitrite. 



The true morphological nature of these supposedly normal 

 looking parthenogenetic larvae of annelids has been clearly eluci- 

 dated by Lillie ('02), who has shown that the unfertilized eggs of 

 Chaetopterus, after exposure to salt solutions, pass, without segmen- 

 tation, through certain phases of differentiation, resembling some 

 of the normal processes, although the resulting ciliated structures 

 are widely different from trochophores arising from fertilized 

 eggs. Since my results in Thalassema are utterly unlike those of 

 Lillie, it is necessary to refer to his observations more fully. 



