388 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



embryo. The body is visible between the two halves. The embryo now 

 undergoes its first molt, after which it assumes a horseshoe form, the head 

 not quite touching the tail, the dorsal side directed outwardly, the body 

 nearly cylindrical although somewhat thicker in the middle owing to the 

 presence of the greater quantity of yolk. At this time the further 

 development of the appendages and of the internal organs, later to be 

 described, is taking place. A second molt then occurs with an increase in 

 length but a decrease in diameter of the embryo, whose middle section is 

 still more or less circular but with the anterior and posterior ends of the 

 body already assuming their definitive flattened (depressed) form. This 

 stage is still under the care of the mother whose body encircles the brood. 

 The stage that is intermediate between the strictly embryonic and the 

 free-living has been designated by Latzel as the "fetal stage." The 

 young colorless fetus is helpless, nearly motionless, at most capable of 

 creeping slowly over the body of the mother, and still dependent upon 

 the yolk within it for sustenance. After a third molt the free-living 

 centipede assumes its definitive shape and leaves its mother to forage for 

 itself, the reproductive organs maturing during the succeeding instars. 



Before the first molt has taken place, an epithelial thickening will be 

 observed along the ventral margin of the anlagen of the sternites (Fig. 

 344, ggl). From this thickening an inwardly and medially directed 

 prohferation of ganglion cells occurs. As development proceeds, the 

 sternites of each segment are formed from the two lateral anlagen and 

 from the membrana ventralis, the ganglion masses being carried mesad to 

 unite into a single strand (Fig. 345). The contraction of the membrana 

 ventralis causes its flat cells to become closely crowded, rounded cells. 

 The tergites likewise are composed of three parts: the two lateral parts, 

 anlagen of the tergites, and the median membrana dorsalis, the lines of 

 demarcation of the three parts becoming obliterated. This obliteration, 

 however, is a transitory condition, since later, before the first molt, a 

 suture appears on each side of both tergite and sternite, the middle dorsal 

 section owing its origin largely to the membrana dorsalis, the ventral to the 

 membrana ventralis. In addition to the longitudinal sutures, a transverse 

 suture is formed close to the anterior margin of each tergite; this suture, 

 in general, outwardly marks the location of the dorsal longitudinal 

 muscle insertions. Likewise before the first molt the tracheal invagina- 

 tions are formed just above the base of the rudimentary appendages, 

 where the integument will remain soft and flexible to form the pleural 

 membrane (a derivative of the tergum). 



Meanwhile at the anterior end the primitive preoral acron is joined by 

 the forward migrating postoral preantennal and antennal segments. The 

 unpaired clypeus is derived from the acron, the clypeus bearing the 

 unpaired labrum on its anterior margin. The acron lacks a coelomic sac. 



