A TYPE OF EMBRYONIC DEVELOPMENT IN INSECTS 



19 



the yolk and above and upon the splanchnic mesoderm bands (Figs. 19, 

 20, splm). From both fore and aft the paired enteron ribbons grow 

 toward each other toward the middle of the embryo where they fuse. 

 In some insects at the same time numerous cells that were set free into 

 the epineural sinus from the middle strand attach themselves to the yolk 

 between the ribbons. By growth in width of the ribbons and by tan- 

 gential division of the cells liberated from the middle strand that now 

 lie between the ribbons, a single median sheet is formed which late in 

 embryonic life grows around the yolk on both sides and over the top to 

 form the tube that constitutes the mid-gut epithelium. According to 

 Hirschler, not all of the anterior cell mass is used up in the formation of 

 the mid-gut epithehal ribbons; the anterior part migrates forward and 



mge 



Fig. 21. — Cross section of developing heart, (cbl) Cardioblast. (div) Dorsal diverticu- 

 lum, (ect) Ectoderm, (eps) Epineural sinus. (/) Fat. (mes) Remains of coelomic wall. 

 (mge) Mid-gut epithelium, {pd) Pericardial septum, {sin) Blood sinus of gut. 



becomes paired to form a spherical mass on each side of the stomodaeum. 

 This pair of spherical masses is the subesophageal body which most 

 embryologists believe to be of mesodermal origin. At the blind end of 

 the proctodaeum three pairs of evaginations arise which elongate to 

 become the Malpighian tubules. Shortly before hatching, the blind ends 

 (hmiting membrane) of the stomodaeum and proctodaeum break down, 

 and thus continuity of the alimentary canal is established. 



Circulatory System, Pericardial Cells, Pericardial Septum, and the 

 Paracardial Cell Strand. — Laterodorsally, where the splanchnic and 

 somatic mesoderm meet, the cardioblasts (Fig. 20, chl), or heart cells, 

 develop. These strands of cardioblasts, one right and one left, are pushed 

 dorsad by the dorsad growth of both the ectodermal body wall and the 

 mesoderm until the two strands meet and fuse into a tubular heart 

 (Fig. 21). The cardioblasts do not all meet simultaneously; but in some 

 instances, as in Donacia, the tube first closes ventrally and later dorsally 

 at the posterior end of the heart, whereas elsewhere the tube fuses first 

 dorsally and then ventrally. The aorta is not formed by cardioblasts 

 but from the median walls of the antennal coelomic sacs. The aorta 

 thus develops independently of the heart; the dorsal blood vessel in its 



