470 F. A. I'OTTS. 



or so of eggs have been expelled from the uterus. In such a 

 case of course the diminution in fertility is due to tiie small 

 amount of nourishment supplied to the ovary, which is only 

 enaljjed to ])roduce a liuiited number of eggs. AMien a 

 cloudy film of bacteria is seen at the bottom of the culture- 

 drop the conditions are exceptionally favourable for the 

 growth of the nematodes, and fei'tile eggs are laid i-apidly till 

 the spcrmato/iOa are exhausted. IF, instead oP peptone, a 

 saturated solution of gelatin be used ;is ji culture-medium, a 

 vevy different effect is produced. For the first day or so after 

 a woi-in is moved from a peptone solution into gelatin the 

 rate of egg-production is fairly maintained, but afrerwai'ds it 

 sinks very low indeed, though the life of the ])ai"ent and the 

 period of fertility is much longer than that of individuals in 

 peptone. Thus, for instatice, for two hermaphrodites of the 

 same generation bred in peptone but kept during maturity in 

 peptone and gelatin respectively, the following figures were 

 obtained : 



(1) Peptone. (2) Gelatin. 



Sept. 2nd-4th, 28 eggs. Sept. 2nd-4th, 19 eggs. 



„ 4th-5th, 32 „ „ 4th-15th, 17 „ 



„ 5th-6th, 21 „ 



„ Cth-7th. 20 „ 



Total for 5 days 101 eggs Total for 13 days 36 eggs 

 When a second generation of Diplogaster maupasi is 

 raised in gelatin, when about twenty fertile eggs have been 

 pi-oduced the uterus contains sterile disorganised ova. It 

 appears from this that the effect of the substitution of gelatin 

 as a foodstuff is not merely to curtail the formation of eggs 

 in the ovary, but also to very considerably limit the number 

 of spermatozoa produced. 



Though under favourable conditions the average fertility 

 varies between two and three hundred in the majority of 

 species now known, there are undoubtedly some which 

 normally produce a very much smaller number of offspring. 

 In the summer of 1907 I had under observatiou a species of 



