PLATE U— concluded. 



Fig. 21. — Diagram of a Seginent of an Embryo. 



(After Heymons. ) 



Figure 20 shows that the sternellum is often obsolete, so that the furcae 

 appear to arise from the caudal margin of the segment. 



Figure 21. — Heymons has shown that in a comparatively early em- 

 bryonic stage each segment of the body is composed of three parts: a 

 median field (mf), and two lateral fields (If), and that the appendages 

 are developed as evaginations of the lateral fields {Ig). 



Figure 20. — The abdominal sternites of the adult gryllus show the 

 lateral elements of the sternites, the portion lying between the appendage 

 and the median field (//). 



As a rule each sternite is an undivided sclerite. 



Fig. 22. — Ental Surface of the Pleiirites of the Meso- and Meta-thorax 

 of Melanoplus, Showing the Lateral Apodemes. 

 The lateral apodemes show the line of union of the subsegments on the 

 pleural aspect of a thoracic segment. 



Fig. 23. — The Base of a Leg of a Cockroach. 

 This shows the relations of the appendages to a typical segment: 

 X, Point of pleural articulation of the coxa, ventral end of the foot of 



the lateral apodeme of the segment, the ventral end of the episternum 



(es), and epimeron (em). 



y, Ventral articulation of the coxa; sp, spiracle; ap, apodeme; ac, ante- 



coxal piece; 2ac, second antecoxal piece; tr, trochantin; es, episternum; 



em, epimeron. 



