402 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



At an earlier stage in both pregenital and genital segments, as in the 

 rump segments, the eoelomic sacs in large part lie in the evaginations of 

 the rudiments of the appendages. Later they are drawn more and more 

 into the body cavity where one may distinguish their thickened somatic 

 and the much thinner visceral walls. Muscles develop from the somatic 

 walls, while the visceral wall divides the eoelomic sac from the large, now 

 yolk-free body cavity of the pregenital and genital segments. The large 

 eoelomic sacs of these segments are contiguous on the middorsal line 

 (Fig. 359), but ventrally they reach only the margin of the nerve cord. 

 After the separation of the muscle anlagen the somatic walls have also 

 become thin. At the lower ends of the sacs four diverticula (Fig. 359, 

 amp) are given off: two for the pregenital and two for the genital segment. 



amp 



Fig. 359. — Scolopendra cingulata. Cross section of genital segment, (amp) Genital 

 ampulla, (ff. coel) Genital coelome. {ggl) Ganglion, (h) Heart, (mus) Muscle, (pm) 

 Pericardial membrane. (Adapted from Heymons.) 



These diverticula, which we may call "genital ampullae," are the remains 

 of the eoelomic cavities that extended into the rudimentary appendages 

 of the corresponding segments. The breaking down of the walls between 

 the two sacs on each side and then of the median wall gives rise to a single 

 large thin-walled sac, or genital sinus. The single dorsal genital sinu5 is 

 produced ventrally into two sac-like shanks which are in communication 

 with the ampullae. The wall between the genital sinus and the tubular 

 gonad now also breaks down, establishing continuity. The genital 

 sinus with its shanks then becomes more constricted and inconspicuous, 

 the latter becoming tubular canals (ducts) connecting dorsally with the 

 gonad and ventrally ending in the ampullae. Toward the end of the 

 embryonic period the caudal end of the gonad (genital tube) passes over 

 on the right side of the alimentary canal finally to assume a ventral 

 position. In consequence of this shift of position the ducts noted above 

 are much affected. The right duct remains relatively large and thick 



