i68 Embryonic Dcvclopniciit of Cluronounis 



rotates on its principal axis tlirougli an angle of i8o", and 

 the parts corresponding to the future head and tail, which 

 lay on the flat side of the o^^^^ are shifted to the opposite, 

 or convex side. The rotation is effected in about a quarter 

 of an hour. The embryo of Simulium effects a similar 

 rotation. In some Orthopterous eggs with copious yolk 

 the embryo travels from the ventral to the dorsal surface, 

 always returning to its original position before hatching ^. 

 The embryo of Chironomus, too, regains its original 

 position by a second rotation. 

 Formation Early in the second day segmentation of the ventral 



of S 6 O"!!! Gil t S . 



anciappen- plate sets in. The three jaw-segments are first defined 

 ( ages. ^^^ 125). A little later the brain section of the head, in 

 front of the jaw-segments, sends out a pair of lateral lobes, 

 which almost touch in the middle line. The central 

 unpaired lobe projects a little further forwards than the 

 lateral lobes, and is a good deal smaller. From the central 

 lobe will be developed the clypeus and labrum, while the 

 lateral lobes will yield the epicranial plates ; the rest of 

 the body then rajjidly segments from before backwards, 

 until three thoracic and nine abdominal segments are 

 developed^. 



Somewhat later, and after the formation of the rudi- 

 ments of the nervous system, paired buds appear, first on 

 the jaw- segments, and a little later on the first and last 

 abdominal segments ; these are the first signs of the 

 appendages (fig. 125). 



It seems that the full number of insect-segments is 

 about twenty. Of these the first three are supposed to 

 be indicated chiefly by the divisions of the brain ; only 

 one, the second, bears a pair of appendages, the antennae ; 



' Wheeler, 1893, p. 68. 



^ The greatest number of abdominal segments clearly seen in any 

 insect is eleven. Indications of a transitory premandibular segment 

 have been detected in some insects, but not, so far as we know, in 

 Chironomus. 



