22 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



am 



later stages there may be a considerable reduction in number owing to 

 fusion of the brain ganglia, the subesophageal ganglia, and, in more 

 highly specialized insects, thoracic and certain abdominal ganglia also. 



Tracheal System. — The stigmata of the respiratory system appear 

 shortly after the outer neural ridges have become segmented — a pair 

 each for mesothorax and metathorax and a pair for each of the first eight 

 or nine abdominal segments. Each of the stigmata is the mouth of a 

 tracheal invagination. In most insects this 

 trachea forks, the posterior branch of one fork 

 fusing with the anterior branch of the next 

 succeeding one, thereby forming two major 

 tracheal trunks. Transverse branches arise 

 which connect the right and left sides, and 

 from the minuter subdivisions the tracheoles 

 develop. In the more primitive insects longi- 

 tudinal trunks are not formed. 



Oenocytes. — The oenocytes, a clump of 

 large rounded cells, may be observed in the 

 ectoderm at the time the tracheal invagina- 

 tions first appear, a little behind and laterad of 

 them. These cells occur in all abdominal seg- 

 ments except possibly the last one or two. As 

 the embryo grows older, the oenocytes extend 

 as a more or less connected strand of cells into 

 the interior adjacent to the tracheal trunks 

 and press into the fat body. Eventually they 

 lose their ectodermal connection. 

 Migration of Germ Cells. Gonads. — The germ cells are early differ- 

 entiated as a group of cells located at the posterior pole of the egg. When 

 the posterior amniotic fold develops, they pass through the primary 

 epithelium (blastoderm) and lodge between this and the yolk (Fig. 23). 

 By the time the proctodaeum and the coelomic sacs appear, the germ 

 cells have migrated forward between the entoderm and the yolk and have 

 divided into two masses. In about the ninth or tenth abdominal segment 

 one mass migrates to the right, the other to the left, penetrating the 

 mesoderm. When the coelomic cavities have appeared, the germ cells 

 pass further forward, penetrate into the genital ridge, increase in number, 

 and form a mass of cells surrounded by the tissue of the genital ridge, 

 the anlage of the gonads. 



Secondary Dorsal Organ and the Dorsal Closure. — As soon as the 

 embryo has attained its maximum length and is about to begin shorten- 

 ing, amnion and serosa fuse longitudinally for a short distance at the 

 head, as in Forficula (Fig. 24A-D), whereupon a longitudinal fissure 



Fig. 25. — Sagittal section. 

 Formation of secondary dorsal 

 organ (do) from the serosa. 

 {am) Amnion forming a pro- 

 visional dorsal closure, (y) 

 Yolk. 



