12 EMBRYOLOGY. 



ovary 1 . The eggs from the ovary develope parthenogenetically 

 within the oviduct, and so long as there is plenty of food and warmth 

 the generations produced are always parthenogenetic forms. The 

 failure of warmth and nutriment causes the production of true males 

 and females, and so the cycle is completed. We must suppose that 

 the capacity possessed by so many female insects of producing eggs 

 capable of developing without the influence of the male element, has 

 been, so to speak, taken hold of by natural selection, and has led to 

 the production of viviparous parthenogenetic forms, by which, so long 

 as food is abundant, a clear economy in reproduction is effected. The 

 continuance of the species during winter is secured by the production 

 of males and females, the females laying eggs in autumn which are 

 hatched in the spring. 



In Chermes there is less modification of the primitive condition 

 in that the parthenogenetic generations lay their eggs like the 

 impregnated females. In the gall-flies (Cynipidae), there is frequently 

 an alternation of generations of the same kind as in Chermes ; there 

 being no viviparous forms. The individuals of the different genera- 

 tions ditfer from each other to some extent in all these cases. 



A second type of alternations of parthenogenetic and sexual 

 generations is exemplified by the cases of Chironomus and Cecidomyia, 

 where the larvw which develope from the eggs of the fertilized 

 female produce parthenogenetically, by means of true ova, forms 

 which eventually after several generations (Cecidomyia) of larval 

 reproduction give rise to sexual forms. The explanation is here 

 practically the same as in the case of Aphis, and is paralleled in the 

 gemmiparous series by the production of buds in the larval forms of 

 Trematodes, etc. A very similar occurrence takes place in Ascaris 

 nigrovenosa (vide chapter on Nematoidea), except that larval forms, 

 which carry on reproduction and then perish without developing 

 farther, do so by a true sexual process. Thus there is an alternation 

 of generations of adult and larval sexual forms. The Axolotl is an 

 intermittent example of the same phenomenon. 



As might be anticipated from the mode in which alternations of 

 generations have become established, incomplete approximations to it 

 are not uncommon. Such approximations are especially found in the 

 Arthropoda, where alternations of sexual and parthenogenetic gene- 

 rations frequently take place, in which the individuals of different 

 generations are similarly organised (Psychidse, Apus, &c.). Another 

 approximation is afforded by the parthenogenetic winter eggs of 

 Leptodora amongst the Phyllopods, which give rise to Nauplius 

 larva?, while the young hatched from the summer eggs do not 

 pass through a metamorphosis. Numerous transitional cases are also 

 found amongst the forms in which there is an alternation of sexual 

 and gemmiparous generations. 



The whole of the cases to which allusion has been made in this 



1 The distinction drawn by Huxley between ova and pseudova does not appear to 

 me a convenient one in practice. 



