Mr Potts, Sexual Phenomena in the Free-living Nematodes. 375 
The cultures produced males in constantly small but fluctuat- 
ing numbers. There were many cases where several generations 
passed before the appearance of a single male, in a whole series 
of cultures. These cases, in which the highest percentage of 
males were observed, may be exemplified by a culture where four 
males and 60 hermaphrodites were counted. In no case was it 
found possible to obtain by selection a strain producing a high 
proportion of males. Artificial alterations in the environment 
apparently made no difference to the sex proportions. Unfavour- 
able conditions were brought about by dilutions of the peptone- 
solutions and by crowding, but these brought about no increase 
in the numbers of the males. 
It may be suggested that the origin of the supplemental 
males is to be looked for in the occasional parthenogenetic de- 
velopment of eggs, but increases in the salinity of the nutrient 
medium, often considerable, effected no alteration in the sex 
proportions. It is well known that an increased osmotic pressure 
in the circumambient medium does often cause the parthenogenetic 
development of unfertilised eggs ( e.g . Echinoderms and Annelids). 
The free-living nematodes offer one of the best exceptions to 
the rule of cross-fertilisation among animals. The species under 
observation multiplied only by self-fertilisation for a year, but 
exhibited no signs of degeneration at the end of that period. 
Distinctly in contrast with this are such cases as Ciona intestinalis, 
where the eggs and spermatozoa of the same individual are said 
to be in general incapable of conjugation 1 . 
1 Castle, “Early Embryology of Ciona intestinalis ,” Bull. Mas. Comp. Zool. 
xxvii. 1896. 
