I08 E. E. JUST. 



intervals after insemination have been studied. The sperm does 

 not enter; or, if it enters must disintegrate early for I have never 

 found sperm nuclei in these preparations. 



Clearly, then, one may not use the eggs of these worms in- 

 discriminately. 



C. Artificial Parthenogenesis. 



The following agents have been used in an attempt to bring 

 about artificial parthenogenesis in the egg of Platyntreis megalops : 



1. Centrifuging, 



2. KCl, 



3. NaOH, 



4. KOH, 

 5- HNO3, 



6. HCl, 



7. Warm sea-water. 



The eggs were cut out of the worms in sea-water centrif uged ; 

 subjected to varying quantities of salt, alkalis, or acids for dif- 

 ferent lengths of time; or warmed in sea-water for from five to 

 thirty minutes at 35° C. These methods gave polar body forma- 

 tion, cytoplasmic changes, fusion of the oil drops, and finally 

 chromatin disintegration in the animal hemisphere. The eggs 

 never cleaved. 



Study of the literature reveals the fact that the clearest cases 

 of artificial parthenogenesis closely simulating the normal in 

 cleavage and in larval development are of those eggs that have 

 formed one or both polar bodies when shed: the echinids, for 

 example, and the asteroids. Other eggs shed in the germinal 

 vesicle stage like those of Polynoe (Loeb '08), Amphitrite (Loeb 

 '01; Scott.) Nereis (Lillie '11), etc., give only differentiation 

 without cleavage or incomplete cleavage. Loeb and Wasteneys' 

 work on Chaetopterus with ox serum as well as Miss Allyn's 

 on the same egg with heat are exceptions. The great exception 

 to the general statement made above is Thalasema (Lefevre) 

 where it appears with single substances, acids mostly, normal 

 development is closely simulated. On the whole, however, 

 ovocytes yield less readily to parthenogenetic agents than 

 mature ova. 



