36 



ENTOMOLOGY. 



There is a remarkable uniformity in the mode of develop- 

 ment of the wingetl insects. In general, after fertilization 

 of the egg, a few cells appear at one end of the egg; these 

 multiply, forming a single layer around the egg, this layer 

 constituting the blastoderm. This layer thickens on one 

 side of the egg, forming a whitish patch called the primi- 

 tive streak or band. The blastoderm moults, sloughing off 

 an outer layer of cells, a new layer forming beneath; the 



;> 



.«** 



ifflwi 



ti 



Fig. 33.— Primitive band or 

 germ of a Sphinx moth, 

 with the segments indi- 

 cated, and their rudiment- 

 ary appendages, c, upper 

 lip; at, antennae; md, man- 

 dibles; nix, nix, first and 

 second maxillae; I, I', I", 

 legs; al, abdominal legs. 



Fig. 34.— Embryo of a Water-beetle (Hi/dro- 

 philus). E, egg; K, head: oh upper lip; »<, 

 mouth; an, antennas; fc 1 , mandibles; t 2 , fc 3 , 

 maxillae; B. thorax; 6,, b 2 . b s , legs; ft|-/t 10 . 

 ten pairs of rudimentary abdominal legs, of 

 which all except fc, disappear before the in- 

 sect hatches; a, anus.— After Kowalevsky. 



skin thus thrown off is called the serous membrane; the 

 second germ-layer (ectoderm) then arises, and a second 

 membrane (called amnion, but not homologous with that of. 

 vertebrates) peels off from the primitive band just as the 

 appendages are budding out, so that the body and appen- 

 dages of theembrvo insect are encased in the amnion as the 

 hand and fingers are encased by a glove. As seen in the 



