152 THE BIOLOGY OF INSECTS 



— the Arthropoda — to which insects belong. The egg shell 

 in most insects is elongate in shape with rounded ends — 

 the eggs {" fly-blow ") of a bluebottle furnish familiar 

 examples, but the most varied forms — spherical, discoidal, 

 cylindrical, flask-shaped — may be seen, and the outer 

 surface of the shell is often marked with ridges and furrows, 

 presenting to the observer a beautifully sculptured aspect. 



The embryo (a common term for the unhatched or 

 unborn young) is built up by an orderly process of division 

 (segmentation) of the egg. The zygote-nucleus (p. 117 

 above) divides in two and the daughter-nuclei divide 

 repeatedly, so that their number rapidly increases. Each 

 nucleus becomes the centre of a protoplasmic mass or cell, 

 though cell-boundaries may not, in the earlier stages of the 

 process, be very evident. As the segmentation of the egg 

 thus proceeds the numerous cells arrange themselves for 

 the most part around the outer region, enclosing a central 

 mass which consists of yolk spheres and corpuscles with a 

 few yolk-cells. Thus there is formed a definite cell-layer 

 or blastoderm surrounding the yolk. Then the blastoderm 

 on one long face of the egg becomes thicker than on the 

 other owing to the deepening of its component cells. The 

 thickened portion or germ-band marks the ventral, the 

 thinner the dorsal aspect of the growing embryo, the 

 inception of which is marked by the insinking of a mass of 

 cells (" middle plate ") along the axis of the germ-band 

 to form a lower layer (endoblast), while the rest of the germ- 

 band overgrows it and becomes the outer embryonic layer 

 (ectoderm) ; thus the embryo assumes a definitely two- 

 layered condition (Fig. 39). Then by uprising of the thin 

 blastoderm around the germ-band or by the inpushing of 

 the latter into the former a protective layer (amnion) is 

 formed over the embryo (Fig. 39, B, C). Meanwhile the 

 ventral embryonic region becomes definitely segmented owing 

 to the successive appearance of a series of transverse inter- 

 segmental grooves. The early embryo may be regarded as 

 consisting of a primitive head and tail ; the remaining 

 segments of the body are formed in succession from before 



