408 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



cells together. These cells then arrange themselves in epithelial fashion 

 and become enlarged and vacuolated (Fig. 364, ent). The musculature 

 of the gut is derived from the mesoderm which surrounds the base of the 



stomodaeal invagination (Fig. 363, 

 mes). With the lengthening of the 

 invagination the mesoderm is carried 

 further inward (Fig. 364, mes), form- 

 ing a provisional connection between 

 fore- and mid-gut. This connection 

 of mesoderm divides into two 

 lamellae which give rise to an outer 

 longitudinal muscle-fiber layer and 

 an inner circular muscle-fiber layer 

 (Fig. 364, mes 1 and mes 2). 



At the time when segmentation 

 takes place, cell proliferation begins 

 at the incisures; the cells wander out 

 of the blastoderm and gather in loose 

 mesenchyme masses on the segments. 

 These masses become more compact 

 and form the mesodermic coelomic sacs (Fig. 365, mes). This process 

 of mesoderm formation begins with the clypeus, where a coelomic 

 sac is formed, and proceeds both forward and backward corresponding to 

 the body segmentation, with coelomic sacs forming in the preoral region 



Fig. 364. — Platyrhacus amauros. Lon- 

 gitudinal section at junction between 

 fore- and mid-gut. (ect) Ectoderm. 

 {ent) Entoderm, {mes 1, mes 2) Outer 

 and inner mesoderm layers. {From 

 Pflugfelder.) 



mes.c 



mes.c 



Fig. 365. — Platyrhacus amauros. Longitudinal section of thoracic "double segment." 

 {mes) Mesoderm, {mes. c) Center of mesoderm proliferation, {v) Vitellophag. {From 

 Pflugfelder.) 



and in the mandibular, maxillary, postmaxillary, and the following seg- 

 ments. As appendages appear, the sacs push into these evaginations, 

 forming outwardly a close union with the ectoderm. The walls of the 



