386 THE ANATOMY OF INVERTEBRATED ANIMALS. 



Leuckart, Metschnikoff, and Ganin/ have shown that these 

 germs are detached from the pseudovarium, which occupies 

 the place of the rudimentary ovarium ordinarily found in larvae ; 

 and that each represents the egg-chamber of an ordinary in- 

 sect ovariole with its epithelial capsule, ovum, and vitelligenous 

 cells. 



In the ordinary process of growth of an insect, from the 

 time it leaves the egg until it attains the adult condition, every 

 marked change in the outward form of the body, or of its ap- 

 pendages, is accompanied by a shedding of the cuticula. In 

 some cases the modification eflFected at each ecdysis is very 

 slight, and the moultings of the cuticle are numerous, amount- 

 ing in a species of Day-fly (Ghloeon), described by Sir John 

 Lubbock, to as many as twenty. In such a case as this, the 

 structure of the adult is gradually substituted for that of the 

 larva, and the organs of the larva, for the most part, pass into 

 those of the adult. 



The like holds good of some insects which undergo meta- 

 morphosis, that is to say, in which a quiescent pupal condi- 

 tion is interposed between the active larval and the active 

 imaginal states. Herold and Newport have described at 

 length the series of changes by which the elongated gangli- 

 onic chain of the Lepidopterous caterpillar is converted into 

 the much more highly concentrated nervous system of the 

 Butterfly ; and Weismann has shown by what gradual steps 

 the apodal Corethra-lsirvn. acquires the character of the Dip- 

 terous imago. But, in the Flesh-flies {Musca)^ and probably 

 in many other members of the division of the Diptera to 

 which they belong, the apodal maggot, when it leaves the 

 egg^ carries in the interior of its bodv certain regularly ar- 

 ranged discoidal masses of indifi'erent tissue, which are termed 

 imaginal disks.^ Of these, twelve are situated in the thora- 

 cic region, two on each side of each thoracic segment, while 

 two others lie in front of the pro-thoracic disks. These imagi- 

 nal disks undergo little or no changfe until the larva incloses 

 itself in its hardened last-shed cuticle, and becomes a pupa. 

 But they then rapidly enlarge ; each of the sternal thoracic 

 disks gives rise to a leg and to its half of the sternal region of 



^Leuckart "Die unsreschleolitliclie Yermelimnsr cler CeoidoTnvienlarven " 

 ( G'ttinqer Nachrichten^ 1805) ; K. von Baer, " Uelier Prof. Nic. "Wasrner's Ent- 

 deckunsr," etc. (" Melansres hioloeiques tires du Bulletin de I'Acad. Imp. des 

 Sciences de St.-P^tersbouror," 1865). 



^ 2 See the remarkable memoir of Weismann, " Die nachembryonale Ent- 

 "wickelung der Musciden." 



