66 THE CLASS OF INSECTS. 



ceals the base of the antennae. It is probable that more 

 careful observation would have shown the end of the abdomen 

 folded back upon the dorsal region, as usual at this period in 

 the embryos of those insects whose embryology has been 

 studied. 



The antennae, mandibles, and maxillas form a group by them- 

 selves, while the second maxillae (or labium) are very much 

 larger and turned backwards, being temporarily grouped with 

 the legs. 



There are traces only of the two basal sterna of the abdo- 

 men. This indicates that the basal abdominal segments gi'ow 

 in succession from the base of the abdomen, the middle ones 

 appearing last. The post-abdomen (Fig. 59 a) has probably 

 been developed synchi^onous with the procephalic lobes, as it is 

 in all insect and crustacean embryos yet observed. As stated 

 by Zaddach, these two lobes in theii* development are exact 

 equivalents ; antero - posterior sjrmmetry is very clearly de- 

 marked, the two ends of the body at first looking alike. But 

 in this stage, after the two ends of the body have been evolved 

 from the primitive cell-layer, development in the post-abdomi- 

 nal region is retarded, that of the head progressing with much 

 greater rapidity. 



In the next stage (not figured) the yolk is completely walled 

 in, though no traces of segments appear on the back or side of 

 the embryo. The revolution of the embryo has taken place ; 

 the post-abdomen being curved beneath the body, and the back 

 presenting outwards. 



The rudiments of the eyes appear as a darker, rounded mass 

 of cells indistinctly seen through the yolk-granules, and situ- 

 ated at the base of the antennae. They consist of a few epithe- 

 lial cells of irregular form, the central one being the largest. 



The second maxillae are a little over twice the length of the 

 first maxillae and are grouped with the legs, being curved back- 

 wards. They are, however, now one-third shorter than the an- 

 terior legs. The second maxillary sternum is still visible. 



The tip of the abdomen (or post-abdomen) consists of four 

 segments, the terminal one being much the larger, and ob- 

 scurely divided into two obtuse lobes. 



The abdominal sternites are now well mai'ked, and the ner- 



