232 LECTURE XVIII. 



(" cambium " of Herold)^ a smooth transparent integument is formed, 

 like that in the Entozoon, first along the ventral aspect, then ascend- 

 ing up the sides to the dorsal aspect, which is likewise closed in by 

 the reci[)rocally approximating folds which cover the cephalic and 

 caudal extremities. The division of the integument into the thirteen 

 segments commences at the ventral aspect, which is convex, the ver- 

 miform body of the embryo being, at first, bent upon the back. 



In the capitate larvse the entozoal type is quickly left by the cer- 

 vical constriction, and the development of a distinct head, which 

 commences by the formation of the part afterwards retained as the 

 labrum. The mandibulae and antennae next appear behind the 

 labrum as convex lobes ; and the part of the head in the lower in- 

 terspace of the mandibles forms the labium. The maxillae next bud 

 forth between the labium and the mandibles, and the median fis- 

 sure, surrounded by the rudimental troplii, sinks deeper into the 

 substance of the head, and, meeting a slender anterior production 

 of the internal vitelline sac or cavity, establishes the mouth and 

 oesophagus. Whilst these stages are in progress, the peripheral 

 series of included vitelline cells have undergone a series of spon- 

 taneous fissions ; whereby the remaining mass becomes included 

 within a second stratum or cambium, which, by coalescence and 

 further metamorphoses of the cells, is transformed into the tunics of 

 the alimentary canal, the intersjiace between which and the outer 

 integument forms the abdominal cavity. A certain proportion of 

 the vitellus, not included in the ellipsoid alimentary canal, has un- 

 dergone transformations, by which the foundations of the muscular 

 system, the ventral nervous chord, and the dorsal vessel, are laid. 

 An attenuated postei-ior prolongation of the ellipsoid vitelline or 

 alimentary sac forms the rectum, and opens upon the thirteenth 

 segment, while it is bent upon the dorsal aspect. 



In such a condition, but without the cephalic and trophal develop- 

 ment, the entozoiform larva of the flesh-fly is born or excluded from 

 the parent : in a similar condition the larva of the bee quits the vermi- 

 form ovum, but without the external communication with the digestive 

 or vitelline sac, having been established at the posterior extremity. 



In some Coleoptera development proceeds to the formation of the 

 appendages of the head, as above described, and a capitate but apodal 

 larva is excluded, as in the nut-weevil. 



In the other Coleoptera, as the Donacice*, the ventral arcs of the 

 second, third, and fourth segments, send out bulbous rudiments of the 

 thoracic legs, before the tergal or notal elements of the segments are 

 comi)leted ; the abdomen is closed above whilst the development of the 



* Koiliker, loc. cit. p. 15. 



