LARVAL DEVELOPMENT. 547 



external and internal membrane, the former being called the serous 

 membrane, and the latter the amnion (fig. 455, d, e). 



Simultaneously with the above-mentioned appearance of the 

 membranes (in other cases at an earlier stage of development) the 

 ventral plate becomes divided into two symmetrical halves, the 

 germinal bands, which become divided by transverse constrictions into 

 segments (up to 17). First of all three cephalic segments, on which 

 the oral appendages are subsequently developed, make their appear- 

 ance behind the procephalic lobes, which bear the first rudiments of 

 the antennae. Behind these the rest of the primitive segments 

 (mesoblastic somites} are successively marked off. 



Inasmuch as the germinal bands become strongly contracted, their 

 dorsally bent round, terminal portion becomes drawn more and more 

 towards the lower part of the egg, while their lateral parts gradually 

 grow round the yolk to form the dorsal surface of the embryo 

 (fig. 455, f, g, h). With these changes the body of the embryo 

 has assumed a closed form ; it now possssses mouth and anus, the 

 first rudiments of the internal 

 organs and the external appen- 

 dages of the segments, and is soon 

 ready to escape from the egg and 

 begin its independent life. 



In Hydrophilus and the Phry- FIG. 456. JEmchna larva with rudimentary 



ganidcBj peculiar differentiations 



appear on the dorsal surface, giving rise to a dorsal plate, which later 



on becomes folded, so as to form a dorsal canal (fig. 455, i). 



The post-embryonic development takes place, as a rule, by means 

 of metamorphosis, the form, organization and mode of life of the 

 young animal, after hatching, being different from that of the 

 sexually adult animal. It is only in the lowest forms, the partly 

 parasitic Aptera, both sexes of which are without wings, that the 

 young leave the egg as perfect animals (Insecta ametabola). 



In those insects which pass through a metamorphosis, the manner 

 and degree of the transformation differs greatly, so that the dis- 

 tinction of a complete and an incomplete metamorphosis, which was 

 formerly employed, seems to be in a certain degree justified. 



In the case of the incomplete metamorphosis (Rkynchota, Orthoptera) 

 the development of the larva into the perfect winged insect presents 

 a number of stages, during which the larva is capable of free locomo- 

 tion and of nourishing itself. During these stages, which are marked 

 by successive ecdyses, it gradually acquires wings and increases in size, 



