NOTES ON THE FREE-LIVING NEMAT'ODES. 469 



position the spenna-tozoa become exhausted, but it is difficult 

 to obsei"ve exactly wheu the limit has beau reached, because 

 the hi'st laid uufeftilised eggs undergo a kind of incipient 

 parthogenetic developmeut. Such eggs possess a shell like 

 fertiUsed eggs and they complete a few divisions, but the 

 blastomei-es are more regular and equal than in normal seg- 

 mentation; the egg-substance appears greatly shrnuk, sotliat 

 a wide space occurs between it and the egg-shell. 



An examination of the table of descent of Diplogaster 

 mau[)asi will show how widely the fertility varies in a 

 single species even under apparently uniform conditions. A 

 few entries may be specially quoted here for comparison, each 

 pair of individuals being taken from the same generation of 

 nearly i-elated strains and supplied with the same nourishment : 



(1) 12th generation October 20th-25th, 257 eggs. 

 r2th generation October 18th-22nd, 153 eggs. 



(2) 14th generation October 2oth-31st, 143 eggs. 

 15th generation November lst-5th, 285 eggs. 



In this case a parent with low fertility gave in the 

 next generation exceptionally prolific offspring. 



(3) 14th generation October 25th-31st, 192 eggs, 

 15th generation JSTovember lst-6th, 229 es'S's. 



Other cases fall within the wide limits indicated above, so 

 that it may be concluded that under favourable conditions a 

 hermaphrodite individual of D. maupasi will lay 140-290 

 eggs. It is not pretended that such figures as these prove that 

 it is impossible to select strains characterised by high and low 

 fertility respectively, but as far as my observations go, there 

 is a fluctuating variability, not governed by the laws of 

 descent nor always directly traceable to minor changes in the 

 environment. 



The influence of external conditions is, however, very great, 

 and especially is this the case with nutrition. In peptone 

 solutions of every kind, the number of eggs laid depends 

 upon the development of bacteria in the culture-medium. 

 When the peptone is fairly sterile the nematode only lays 

 eggs at long intervals, and eventually dies when only a score 



VOL. 55, PART 3. NEW SERIES. 31 



