158 THE BIOLOGY OF INSECTS 



third views, as set forth here, by supposing that the cell- 

 groups at the inner ends of the two ectodermal inpushings 

 are originally endodermal rudiments which are pushed 

 inwards by the growth of the fore- and hind-gut rudiments 

 of which they appear to form part. It is well known to all 

 students of animal form and development that, largely 

 through the influence of E. Haeckel (1877) and others very 

 great stress has been generally laid on the *' germ-layer 

 theory," according to which the two primary germ-layers 

 (ectoderm and endoderm) give rise throughout the animal 

 kingdom to certain definite regions and systems of the 

 embryo. As regards the ectoderm and its derivatives insects 

 appear to follow the general rule, but on any possible inter- 

 pretation of the facts, there is some abnormality in the origin 

 of the endoderm and its derivatives ; this important layer 

 can be recognised only with great difficulty if at all. Heymons, 

 who finds the endoderm in the yolk cells of the embryo of 

 the primitive bristle- tail, believes that in insects generally 

 the original endodermal mid-gut has been replaced by a 

 new one of ectodermal origin. The dragon-fly embryos, 

 in which TschuproflF states that the central region of the 

 mid-gut is derived from yolk-cells and the front and hind 

 regions from ectoderm, might be regarded as indicating 

 a transition between the. old and the new conditions. 



As the embryo continues to develop, the tissues and organs 

 which in fully formed insects are dorsal in position, tend to 

 grow away from the primitively ventral germ-band, and the 

 yolk becomes surrounded by the cells that form the lining 

 of the gut. The series of appendages : feelers, jaws, legs, 

 assume to some extent their characteristic shapes before the 

 young insect is hatched ; at their origin all are much alike, 

 but they become diff"erentiated for various functions as 

 growth proceeds (Fig. 40). It is of much interest to notice 

 that in many insect embryos (that of the grasshopper Xiphi- 

 dium, for example) small paired limbs appear on many of the 

 abdominal segments, but vanish before hatching (Fig. 40, C, 

 ap). There may be small or vestigial head-appendages 

 between the feelers and mandibles, and rarely also (J. W. 



