314 D . D. WHITNEY. 



Shull and after fertilization always develop into females but 

 neve*r into males. Thus the fertilization causes a change of the 

 sex in the resulting individual from such eggs. All of the func- 

 tional spermatozoa must therefore be identical in their influence 

 in producing females from eggs that if unfertilized would other- 

 wise have produced males. 



SUMMARY. 



1. Dimorphic spermatozoa are produced and shed by the 

 rotifers, Brachionus mulleri, Asplanchna amphora, Polyarthra 

 platyptera, Hydatina senta, Diglena catellina, Euchlanis dilatata, 

 Metopidia lepadella, Brachionus urceolaris and Brachionus bakeri. 



2. One class of the spermatozoa is composed of large, vermi- 

 form, and motile spermatozoa and the other class is composed 

 of small, spindle-shaped and im motile spermatozoa. 



3. The number of motile spermatozoa produced is exactly 

 twice the number of immotile spermatozoa produced. 



4. The motile spermatozoa probably develop from the cells 

 formed by the second division of the normal spermatocytes but 

 the immotile spermatozoa probably develop directly from the 

 rudimentary cells formed by the first division of the sperma- 

 tocytes. 



5. The motile spermatozoa are probably functional but the 

 immotile spermatozoa are probably rudimentary and functionless. 



6. The functional spermatozoa are all identical in their power 

 of determining the sex of the individual that develops from a 

 fertilized egg. After a functional spermatozoon has fertilized a 

 parthenogenetic male egg the egg eventually develops into a fe- 

 male individual instead of developing into a male individual 

 as it otherwise would have done if it had not been fertilized. 



ZOOLOGICAL LABORATORIES, 



THE UNIVERSITY OF NEBRASKA, 

 LINCOLN, NEBRASKA, 

 June 22, 1917. 



LITERATURE CITED. 

 Von Baehr, W. B. 



'08 Ueber die Bildung der Sexualzellen bei Aphididae. Zool. Anz., Vol. 33. 

 Gruvel, A. 



'05 Journ. Roy. Micro. Soc., p. 34. 



