OLIGONEPHRIDIA 



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typical for related insects. The germ band soon becomes long and rather 

 flat, though multilaminar, due to the superimposed condition of the 

 ectodermal cells. The embryo assumes a characteristic S shape, with the 

 cephalic lobes flexed sharply dorsally, so that this region lies directly in 

 contact with the mandibular region (Fig. 213). Typical segmentation 

 furrows have appeared as transverse ectodermal invaginations. The 

 definitive mesoderm layer seems not to be involved in the early segmenta- 

 tion. Appendages arise as paired ectodermal thickenings. At first the 

 mandibular, maxillary, and labial swellings are equal in size to the 

 thoracic evaginations. The pleuropodial rudiments are indistinguishable 

 from the thoracic evaginations, but the abdominal anlagen are insignifi- 

 cant, appearing more like segmental swellings. Mesodermal tissue 

 invades the ectodermal evaginations very early. 



Fig. 213. — Hesperoctenes. Sagittal section, (om) Amnion, (ceph) Cephalic region. (p2) 

 Second thoracic leg. {pleur) Pleuropodium. {tel) Telson. {From Hagan.) 



Just before revolution of the embryo the embryonic envelopes are 

 intact except that the amnion is not visible opposite the pleuropodia. 

 About the time the prothoracic appendages are almost half the body 

 length, the embryo becomes considerably shorter and broader. At this 

 time the embryo undergoes a reversal, so that when it again comes to 

 assume a position in which its length lies parallel to that of the mother, its 

 cephalic region is directed forward in the maternal body. After revolu- 

 tion the embryonic envelopes rupture and become located anteriorly. 

 This is accompanied by a dorsal growth of the body walls and their fusion 

 along the middle line, excepting in the head region. Up to this time there 

 is no sign of a mesenteron. Stomodaeal and proctodaeal invaginations 

 are irregularly one-layered, the latter with four Malpighian tubules. 

 Aggregations of cells at the inner extremities of the stomodaeal and 

 proctodaeal invaginations suggest the presence of formative entodermal 

 cells. Similar cells scattered through the formative haemocoele presage 

 the beginning of the definitive mesenteron. Hagan did not follow the 

 formation of the mid-gut epithelium. About the time of the revolution 

 of the embryo the pleuropodia invaginate (Fig. 214, pleur) and extend 



