208 Prof, von Siebold on the Law of 



appearance in a marked manner until after the fourth change 

 of skin. From Mecznikow's very accurate embryological 

 investigations on insects, it appears also that although the 

 tracing out of the sexual glands takes place very early in all 

 embryos of insects, their further development does not advance 

 at an equal rate in all such embryos ; so that it is only in 

 certain insects that the differentiation of the sexual organs 

 occurs very early, and, indeed, already in the embryo, whilst 

 in other insects, on the contrary, it is postponed, and takes place 

 only in the excluded larvae. In the very young larvae of 

 Simulm J just escaped from the eggy Mecznikow* observed a 

 small round genital rudiment, and concluded from this that 

 the rudiments of the sexual organs are formed in the larvae 

 within the egg. The same author recognized, even at the 

 first formation of the embryo in the viviparous Aphides, the 

 first rudiments of the sexual apparatus as the so-called genital 

 hillf. During the further development of the embryo, and 

 indeed very early, this genital rudiment becomes differentiated 

 into ovarian tubes, in which so-csil\ed pseudova are likewise 

 very soon developed ; so that even during the embryonal life of 

 the aphis-embiyo the development of the new generation com- 

 mences, and goes so far that in the embryos ready to be born 

 two germ-chambers occur in each ovarian tube, of which the 

 lowest already encloses an embryo in the first stage of its deve- 

 lopment!. In Aspidiotus Neriiy on the contrary, Mecznikow§ 

 could not find any genital hill so early produced and differen- 

 tiated into ovarian tubes, such as he had succeeded in dis- 

 covering in the Aphides. 



From these known circumstances in the first development 

 of the reproductive organs of insects it appears that differences 

 occur in it, and that in a certain series of insects the differen- 

 tiation of the sexual apparatus occurs in the embryos while 

 still enclosed in the egg-shell, whilst in other insects this dif- 

 ferentiation only takes place after the exclusion of the larvae. 

 Landois's theory can certainly find no application to the insects 

 belonging to the first series — namely, the Lepidoptera and 

 Flies (Musctdce) ; in the second series, in which Corethra^ Si- 

 midia, and Aspidiotus are to be placed, it may be possible 

 that the still rudimentary and indifferent sexual glands of the 

 larvae are further developed in accordance with the male or 

 female type, under the influence of the incepted nourishment. 

 When, and in what manner in the larvse of the bees the first 



* " Embryologivsche Studien aii Insecten/' Zeitsch. fur wiss. Zool. Bd. 

 xvi. p. 40'5. 



t Ibid. p. 444, pis. 28 and 31. figs. 15-87, and p. 458. 



X Ibid. p. 4o9, pi. 31. % 46. § Ibid. p. 473. 



