ORIGIN OF THE BODY-SEGMENTS 



543 



In the completely segmented primitive band may be distinguished 

 two regions of a peculiar appearance (Figs. 515, 527), one at the 

 anterior, and the other at the 'hinder end. The anterior, the pri- 

 mary head-section, contains the mouth-opening, and is characterized 

 by its lateral expansions, or procephalic lobes. The other section, 

 or posterior section, the so-called anal segment or telson, contains 



FIG. 523. Diagrammatic cross-section through three successive stages of Gryllotalpa. showing 

 the formation of the heart. (Compare Fig. 505.) The germs of the glandular intestinal layer 

 (darmdrHaonblatf) are omitted. A, earliest stage ; the primitive streak extends from *xtoy*. The 

 embryonal membranes are torn and pressed against the back : am, edge of the rent ; rp, dorsal 

 plate '(serosa) ; I, lamella (amnion turned up) standing in connection with the ectoderm of the primi- 

 tive streak. B, second stage ; the primitive streak has completely grown around the yolk ; the 

 dorsal organ is absorbed. C, third stage, dorsal portion ; the formation of the heart is finished : 

 am, vestige of the amnion-fold ; ?>a, blood-sinus ; dd, rudiment of the dorsal diaphragm ; dv, ven- 

 tral diaphragm (compare Fig. 505) ; do, yolk ; dz, yolk-cells ; ee, ectoderm ; (/r, vascular groove 

 (rudiment of the heart) ; I, lamella of the upturned amnion ; Ik, definite body-cavity ; in, trans- 

 verse muscle ; n, nervous cord ; r, heart ; rp, dorsal plate ; p, splanchnic, so, somatic layer of the 

 mesoderm ; us, primitive seginental cavity ; *x, y*, lateral terminations of the primitive streak. 

 After Korotneff, from Korschelt and Helder. 



the anus. Between the two sections lies the segmented primary 

 trunk-segment, which in insects consists of 17 segments. Of these 

 the three most anterior are those destined to bear the mandibles and 

 two pairs of maxillae; the three following are the thoracic, which 

 are succeeded by 10 abdominal segments, besides the llth or telson 

 (pygidium, or suranal plate). 



