EMBRYOLOGY OF A LEPIDOPtEROUS INSECT. 23 



differentiated ventral plate infolds at a point on the median line 

 about two-thirds from the upper end, and forms a very narrow 

 pocket. The cells composing it look like the rest of the cells of 

 the ventral plate at this time ; they are almost round, and have 

 a lining on one side, made of the grey matter which originally 

 bordered the whole egg, but which became a part of the blastoderm 

 cells. The pocket remains open but a short time, but there is a long 

 depression at the upper end of the bunch of cells. The mass of cells 

 is soon cut off from the ventral plate, and they are then free in the 

 body cavity, but remain in contact with the ventral plate at the point 

 where they were produced. Later stages show that these cells produce 

 the generative organs. The generative organs thus appear to be pro- 

 duced by an infolding of the ectoderm, or possibly of the blastoderm, 

 before the ectoderm is produced, but from a portion which is later to 

 become ectoderm. The general idea has been that the generative 

 organs in insects are produced from the mesoderm, although Metsch- 

 nikow, as early as 1866, showed for certain insects a different origin." 

 Those further interested in the details of this subject would do 

 well to refer to the writer's chapter on the " Embryology of a lepidop- 

 terous insect," Ent. Record, vol. v., 1895. 



CHAPTER IV. 



PARTHENOGENESIS OR AGAMOGENESIS IN LEPIDOPTERA. 



It is generally necessary, among the Lepidoptera, that the two 

 generative elements should unite before the fertilisation of the ovum 

 can take place, and, since these elements are always developed in 

 different individuals, it follows that copulation between the sexes is 

 necessary for fertilisation, and for the subsequent production of young. 

 It appears, however, that under certain conditions copulation is not 

 necessary to ensure the production of young, since, occasionally, eggs 

 will produce larvse without the union of the sexes, and larvse thus pro- 

 duced have been recorded as developing in the ordinary course into 

 fully matured and fertile imagines. 



It is a well-known fact that, under ordinary circumstances, the 

 eggs of almost all lepidopterous insects undergo certain changes after 

 being laid. Some of these are common both to fertilised and unfertilised 

 eggs, and since they must be looked upon as the outward sign of a 

 change that is taking place within the egg, it is probable that the first 

 changes which take place in the egg, i.e., the very first stages of 

 embryonic growth, are independent of fertilisation. The changes 

 which take place in the unfertilised eggs of some species are much 

 greater than those which take place in others, and there are, as 

 previously stated, cases on record in which development has proceeded 

 so far, that the growth of the embryo has been completed, and a larva 

 has hatched from the unfertilised egg. 



We see, then, that, under special conditions, nature produces 

 progeny from virgin females without the intervention of the male. 

 The production of such progeny among bees has long been known. 



