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 larve. In the very young larve of 
Simulia, just escaped from the egg, Mecznikow* observed a 
small round genital rudiment, and concluded from this that 
the rudiments of the sexual organs are formed in the larve 
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 
hilt. During the further development of the embryo, and 
indeed very early, this genital rudiment becomes differentiated 
into ovarian tubes, in which so-called pseudova are likewise 
very soon developed; so that even during the embryonal life of 
the aphis-embryo 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 Neriz, 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 larve. 
Landois’s theory can certainly find no application to the insects 
belonging to the first series—namely, the Lepidoptera and 
Flies (Muscide) ; in the second series, in which Corethra, Si- 
mulia, and Aspidiotus are to be placed, it may be possible 
that the still rudimentary and indifferent sexual glands of the 
larvee 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 larve of the bees the first 
* “KEmbryologische Studien an Insecten,” Zeitsch. fiir wiss. Zool. Bd. 
xvi. p. 405. : 
+ Ibid. p. 444, pls. 28 and 31. figs, 15-87, and p. 458. 
{ Ibid. p. 459, pL 31, fig. 46. § Ibid. p. 473. 
