VIII 



AETHEOPODA 



251 



" macrosegments," which subsequently are subdivided into the 

 definitive segments ; but as the so-called " macrosegments " do not 

 correspond to one another in value in different insects, it does not 

 appear that they have any morphological importance, or that the 

 appearance of the dividing lines out of their proper order is of any more 

 importance than the late demarcation of the chelicera segment from 

 the procephalic lobe in Arachnida. 



In Donacia the embryonic area is first divided into a proto- 

 cephalic region and a so-called " protocormic " region, which includes 

 all the rest of the body by a single transverse line of division. The 



FIG. 1 99. Three surface views of the embryo of Donacia crassipes when the germinal streak 

 begins to show division into segments. (After Hirschler.) 



protocormic region is then divided by two transverse lines into a 

 "jaw " region, a " thoracic " region, and an " abdominal " region. 

 In each of these regions the definitive segments are first marked off 

 at their respective hinder borders. Thus we find a stage with two 

 segments in the jaw region, the hinder of which is the second 

 maxillary segment; and with two in the thoracic region also, the 

 hinder of which is the segment of the metathorax, which later 

 bears the third leg. This is succeeded by a stage in which the three 

 definitive jaw segments and the three thoracic segments are clearly 

 marked off' from one another, and in which, in the abdominal region, 

 the last three segments are clearly delimited from one another. In 



