394 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



longitudinal muscles {mus) arc found. The intersegmentally arranged 

 transverse ventral body muscles are developed from the somatic layer 

 of the ventral limb of the coelomic sacs. 



From the somatic walls of the coelomic sacs the fat bodies develop in 

 the form of a narrow strand at the sides of the body (Fig. 348). This 

 strand spreads out to occupy the greater share of the body cavity when 

 the yolk has become reduced. "Fat-body" cells are also found in the 

 space at the sides of the heart which later is bounded by the two lamellae 

 of the pericardial membrane (Fig. 349, pm). These cells have sometimes 

 been called "pericardial cells" and are of mesodermic origin. 



Derivatives of the Ectoderm. — Ectodermal spiracular invaginations 

 are in evidence soon after flexure of the germ band at the time when the 

 anlagen of ectodermal glands and hypodermal invaginations for muscle 

 attachments make their appearance. Nine pairs are developed in all, 



Fig. .350.- 



-Scolopendra cingulata. Cross section of a left leg. 

 Ganglionic pit. (p) Leg. 



{coel) Coelomic sac. {ggv) 



belonging to rump segments 3, 5, 8, 10, 12, 14, 16, 18, and 20. From the 

 simple spiracular invaginations (Fig. 348, st) tubular branches arise which 

 later develop as the primary tracheal trunks. At j&rst one may distin- 

 guish an anteriorly and a posteriorly directed branch and a third some- 

 what smaller one which passes ventrad to the appendage. Later a 

 number of tracheae are added so that finally a tuft of them arises from 

 the bottom of the spiracular pocket. The first and last pairs of spiracular 

 pockets send strong branches into head and tail end, respectively. 

 Taenidial thickenings are first in evidence in the fetus. 



The ventral nerve cord arises as a pair of widely separated longitudinal 

 ectodermal thickenings several cell layers deep before the first molt has 

 taken place (Fig. 344, ggl). Some of the superficial cells in this thicken- 

 ing sink more deeply into the epidermis (Fig. 350, ggv). Here a small 

 shallow depression is formed which is located in the middle of each rump 

 segment ventrad of the rudiment of the appendage. Heymons desig- 

 nated this depression as the "ganglion pit," since it represents the 

 developmental center of the ganglion cells which line the pit. Mesad of 

 and contiguous to the ganglion pits some superficial cells migrate more 



