Lnbreeding 197 
sperm, owing, perhaps, to the absence of some substance in the 
sperm. If, however, the sperm is excited to greater activity by 
alcohol, ammonia, ether, or by certain salts, it may force an 
entrance despite the resistance. 
In another ascidian, Cynthia partita, I have found that self- 
fertilization often occurs, but the sperm generally fertilizes 
its.own eggs less often. than those of another individual. In 
the ascidian, Molgula manhattensis, self-fertilization occurs as 
readily as fertilization by sperm of another individual. 
The failure to self-fertilize in some hermaphroditic or- 
ganisms suggests that factors similar to those in Ciona may be 
at work in forms closely inbred for several generations, where the 
closely related males and females would seem to stand in some- 
what the same relation to each other as the male and female gen- 
erative organs in the same individual. 
LITERATURE, CHAPTER XII 
Bos, Ritzema. Untersuchungen ueber die Folgen der Zucht in engster 
Blutverwandtschaft. Biol. Centralb. XIV. 1894. 
Caste, W.E. The Early Embryology of Ciona Intestinalis. Bull. Mus. 
Comp. Zool. XXVII. 1896. 
Castle, W. E., CARPENTER, F. W., Clark, A. H., Mast, S. O., and Bar- 
rows, W. M. The Effects of Inbreeding, Cross-Breeding, and 
Selection upon the Fertility and Variability of Drosophila. Proc. 
Amer. Acad. Arts and Sciences, XLI. 1906. 
Darwin, C. Animals and Plants under Domestication. 
FABRE-DoMENGUE, P. Unions consanguines chez les Colombins. L’Inter-' 
médiaire des Biologistes, I. 1898. 
Guaita, C. von. Versuche mit Kreuzungen von verschiedenen Rassen der 
Hausmaus. Ber. Naturf. Gesell. Freiburg, X. 1898. 
Morcan, T. H. Self-Fertilization induced by Artificial Means. Jour. 
Exp. Zool. I. 1904. 
Some Further Experiments on Self-Fertilization in Ciona. Biol. Bull. 
- VIII. 1905. 
