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TEXT-BOOK OF ENTOMOLOGY 



yolk, that in insects the invaginated primitive band is the ancestral or primitive 

 one, the overgrown primitive band being derived from it. The overgrown 

 primitive band by its position may also be better insured against certain me- 

 chanical attacks, perhaps also against the danger of drying up. 



B 



C 



D 



E 



FIG. 522. Embryo of Phyllodromia, 15 days old ; revolution about to begin. The stages in revolu- 

 tion are represented, after the rupture of the amnion and serosa, in A to E, which are from 

 embryos 16, 16j, 16J, and 17 days old respectively: as, amnion and serosa; s, edge of serosa; 

 b, dorsal growing- body-wall ; d.o, dorsal organ ; a-, clear zone covered with scattered amniotic 

 nuclei. After Wheeler. 



/. Formation of the external form of the body 



Origin of the body-segments. As we have seen, the first traces of 

 segments appear very early, the primitive band being divided 

 by superficial transverse furrows into segments. This segmen- 

 tation into arthromeres (somites or metameres) can be observed in 

 Hydrophilus and Chalicodoma at a time when gastrulation begins 

 (Figs. 515, 536). The segmentation extends not only across the 

 median portion of the primitive band, through whose invagination 

 the inner layer (endomesoderm) results, but also to the lateral por- 

 tions which become a part of the ectoderm of the primitive band. 

 These transverse furrows correspond to thinner places in the epithe- 

 lium, which in this stage forms the embryonal rudiment. It thus 

 happens that, in the forms named, after the end of gastrulation not 

 only the ectoderm, but also the endomesoderm, is already segmented. 



So early an appearance of segmentation as that observed in Hydrophilus and 

 Chalicodoma we must regard as a falsification of the process of development 

 due to heterochrony. We must consider the conditions observed in other forms 

 as the primitive ones, in which (as, for example, in Lina and in Stenobothrus, 

 according to Graber) the gastrulation and separation of the ectoderm occurs in 

 the still unsegmented primitive band, the division into segments occurring in 

 later stages (Fig. 524). In these forms, then, the segmentation affects the 

 invaginated endomesoderm, as well as the ectoderm. (Korschelt and Heider, 

 p. 789.) 



