541 



INSECTA. 



The germ apparatus, hoAvever, of the so-called Aphide asexual 

 generation not only has a \\-ry great resemblance to the female 

 generative apparatus of insects, but the structure and mode of origin 

 of the germ seems to agree so closely with that of the ovum that 

 the viviparous Aphides must be considered as a peculiarly organised 

 generation of females, the genital apparatus of which has undergone 

 some simplifications adapted to parthenogenesis. However that may 

 be, it will be convenient in this case to call the ovary the pseudovary, 

 and the ova which originate in it and are incapable of fertilization, 

 the pseudova. From this point of view the reproduction of some 



FIG. -153. a, Egg tube of Foificitla. JV.-, Nutritive cells ; E , ovum; OE, epithelium of the 

 wall of the egg tubo. b, Me liau part of the egg tube of a Moth. l?z, nutritive cells of 

 the yolk-chamber ; Ez, ovum in the germ-chamber ; H, connective tissue investment, 

 so-called serosa. c, Egg-tube of Aphis plafanoides with three ovarian chambers (EzEz") 

 and the terminal nutritive chamber with its cells Ni. Dx, yolk cord. 



Diptera (Cccidomyia, Miastor, fig. TOO), which can reproduce them- 

 selves while still in the larval stage, may be explained. 



The development of the embryo takes place as a rule outside tho 

 body of the mother, and occupies a longer or shorter period of time, 

 according to the temperature and the time of the year. The centro- 

 iccithal segmentation leads to the formation of a superficial blastoderm, 

 which surrounds the ovum, and always consists of a single layer of 

 ceils. A part of this blastoderm, on that side ot the ovum which the 

 later history shows to be ventral, becomes thickened and sharply 



