16 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



the appendage-bearing segments of the head but often transversely oval 

 or slit-like in the abdominal segments. Thus the coelomic sacs (coel) 

 are formed, whose dorsal walls will later in part form the splanchnic 

 (visceral) mesoderm; the lateral and ventral walls, which are contiguous 

 to the ectoderm, will form the somatic mesoderm. In Donacia Hirschler 

 (1909) found coelomic sacs in the intercalary and second maxillary seg- 

 ments as well as in the three thoracic and the first nine abdominal seg- 

 ments. In some primitive insects more sacs are present in the head, 

 and sacs are also found in the tenth and eleventh abdominal segments, 

 whereas in the thorax the coelomic cavities extend deeply into the legs. 

 Laterally, the yolk recedes from the mesoderm, resulting in the formation 



neur 

 Fig. 18. — Cross section of embryo. 



neurg 



{am) Amnion. (W) Blood cell, (chl) Cardioblasts. 

 {coel) Coelomic cavity, {ect) Ectoderm, {eps) Epineural sinus. (/) Fat body, {gr) 

 Genital ridge, {neur) Neuroblast, {neurg) Neural groove, {som. m) Somatic mesoderm. 

 {splm) Splanchnic mesoderm. 



of a longitudinal cleft on each side, the beginning of the epineural sinus 

 (Fig. 17, eps). The dorsal part of the inner layer now spHts medially 

 into two lamellae; the dorsal lamella in contact with the yolk becomes the 

 splanchnic layer (Fig. 18, splm); the ventral lamella which passes out- 

 ward to join the fat body (/) becomes the genital ridge (Figs. 17, 18, grr). 

 The lateral- and ventral-lying somatic mesoderm (som.m) likewise differ- 

 entiates into two parts; the part in contact with the ectoderm (Fig. 18, 

 ect) is the anlage which will form the musculature of the body; the 

 mediodorsal part, which has become more loose and spongy, will form the 

 fat body (Fig. 18, /). The large-nucleate cardioblasts, which are found 

 laterally at the junction of splanchnic and somatic layers, will give rise 

 to the heart (Fig. 18, cbl). On the middle Une is the median secondary 

 entoderm strand (Fig. 17, mds) which later will break up into free cells 

 but which now still abuts laterally the anlage of the muscles and fat. 

 Later the lateral margins of the embryo begin to grow around the yolk. 



