THE INSECTS 121 



bands come together and form a single elongated thickening, the so- 

 called 'germ-band'. The midline of the germ-band is the mid-ventral 

 line of the final animal; in insects, it is the ventral side, not the dorsal, 

 which plays the dominant role. 



Along this mid-ventral line, the germ-band folds downwards into a 

 groove whose edges come together to constitute the ectoderm, while the 

 floor of the groove disappears below the surface, as a lower layer or 

 'hypoblast', which eventually gives rise both to mesoderm and to the 

 endoderm of the mid-gut. At the anterior and posterior ends of the germ- 

 band, pockets are pushed in along the length of the embryonic body, 

 forming respectively the mouth and anus, or, more correctly, the stoma- 

 deum and proctodeum. From these, more endoderm is produced, to 

 develop into the foregut and hindgut. 



There is one other important group of internal cells which may be 

 mentioned. In many insects, (particularly in the 'determinate' type to be 

 mentioned below), the cells at the most posterior end of the blastoderm 

 have a special character, distinguishing them histologically from the rest. 

 They are known as the 'pole cells', and they later migrate into the gonads 

 and become the source of all the later-formed gametes. If they are re- 

 moved, by cauterising or otherwise killing them at an early stage, com- 

 pletely sterile individuals may be produced, and it seems that in some 

 forms at least they are the only cells of the embryo wliich are capable of 

 differentiating into gametes. The whole developmental sequence of 

 mother- and daughter-cells, from the pole cells to the final gametes, is 

 sometimes known as the 'germ-line', a term to be carefully distinguished 

 from the 'germ-band' which, as we have seen, is used for the blastodermic 

 thickening from which the embryo arises. 



While the lower layers of the embryo are forming in the way described 



{e),{f),{g) show the oudine of the germ-band, with the longitudinal furrow 

 in which the mesoderm is invaginated (M.F.) and the transverse furrows 

 delimiting the segments: the arrow at the side marks the first interseg- 

 mental furrow, which is between the most posterior head-segment (second 

 maxillary) and the most anterior thoracic; (/j) and (i) show sections through 

 the stages of (e) and {g) at the level of the arrow to show the invaginating 

 mesoderm (dotted) becoming covered by ectoderm (lined). The next stage 

 is shovni in surface view (j) and longitudinal section (fe); the stomodeum 

 (foregut) and proctodeum (hindgut) are pushed inwards from the surface, 

 and from the former endoderm arises and will develop into the midgut ; the 

 appendages are beginning to appear on the segments of the embryo. 

 Figures (/) and (»i) are transverse sections of rather later stages to show how 

 the edges of the embryonic area grow round to enclose the yolk and thus 

 provide the dorsal surface of the completed embryo, which is drawn in (n) 

 (After Seidel 1936.) 



