8 ALTERNATION OP GENERATION. 



quently more difficult of explanation. Indeed, at 

 present no adequate elucidation of the origin of the 

 phenomenon has been given. 



Had the agamic generation at one time a $ ? On 

 this point Adler makes the following highly interest- 

 ing observations. Alluding to the manner of coition, 

 he says, " In repose, with those flies which have a long 

 ovipositor the entire boring apparatus is retained 

 inside the abdomen ; if fecundation took place with it 

 in this position it would be necessary that the penis 

 of the c? was equal in length to the ovipositor rolled 

 in a spiral. But it is infinitely shorter ; further, it is 

 only when the ovipositor is extended that the vagina 

 becomes accessible to the S . Now, as the agamic 

 generations have preserved the habit of extending 

 their organs, does not that indicate that they had at 

 one time males?" (Zeit. f. wiss. Zool., xxxv, p. 236). 

 Again, did the second form originate from the present 

 agamic or bisexual form ? Both lay eggs ; and 

 although we have no direct observation on the point, 

 it may be fairly surmised that the only difference 

 between them, as between other sexual and parthe- 

 nogenetic eggs, is that the former extrudes both and 

 the latter only one of the "polar bodies." The 

 first "polar body" is extruded from all eggs, and in 

 the fertilised eggs the second is replaced by the male 

 sperm. 



Following out his ideas on cyclical generation in 

 the Aphidee, or, as he calls them, the " homoptcres 

 monoi'ques," Lichtenstein regards the agamic form 

 as a larval stage equivalent to the migratory form of, 

 e.g., Phylloxera. 



In his Considerations nouvelles sur la Generation 

 des Pucerons, p. 13, Lichtenstein defines for the 

 Aphidse four forms, as follows : 



The first — the " Souche monoique " — terminates in 

 a large insect, living a long time, always readily recog- 

 nisable by its size, especially with the gallicolous 

 species. 



