NOTES ON THK FREE-l.EVINO NEMATODES. 453 



watch-glass, so that the butch of eggs laid during the pre- 

 ceding twenty-four hours was kept isolated. Each batch 

 was carefully counted to compare with the actual number of 

 individuals attaining adolescence, and in this w;iy records of 

 cultures which gave the actual sex-proportions were distin- 

 guished from others in which mortality before maturity 

 obscured the ti'ue figures. In nny drop culture which con- 

 tained more than about thirty eggs the crowding which 

 ensued was distinctly unfavourable to the chances of 

 survival. 



Precautions were adopted in these experiments to prevent 

 absolutely an association of mature males and hermaphrodites, 

 and so remove Jiny suspicion of cross-fertilisation in the line 

 of descent here followed out. To this end the individual 

 destined to give rise to the next generation Avas separated 

 before any nude had become mature, or else the males them- 

 selves were removed from the culture before the last moult, 

 when they were perfectly recognisable as males, but had not 

 yet assumed the spicular apparatus necessary for internal 

 fertilisation. 



Both sexes become easily distinguishable a considerable 

 time before maturity by the position of the developing gonad 

 and its duct. In the majority of species of Rhabditis and 

 Diplogaster, the vulva opens at the middle of the body of 

 the female, and the gonad is paired, so that the immature 

 hermaphrodite may be recognised by the symmetrical disposi- 

 tion of the clear ovarian rudiments I'ound the middle point of 

 the body. In the male the rudiment of the testis is situated 

 in the posterior half of the body, so that with a little experi- 

 ence it is easy to distinguish a male, even among a, 

 ceaselessly twisting mass of other individuals, by the clear 

 transparent testis running alongside the posterior part of the 

 gut. Sperm-formation begins, it is true, before the last 

 moult. But though the body of the male may contain mature 

 spermatozoa, these can only be conveyed to the hermaphrodite 

 individual by the co-operation of the copulatory spicules and 

 bursa. A young male just before the last moult, at which 



