244 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



tion (Fig. 166C,p?"ocf) in a manner similar to the stomodaeal invagination 

 but at the caudal end of the body. It is first in evidence about 7 hours 

 after the appearance of the stomodaeum. The six Malpighian tubules, 

 of ectodermal origin, first appear in the 117-hour stage. 



The definitive mid-gut, according to Roonwal, arises from the ecto- 

 derm, i.e., from a proliferating mass of cells that come from the ectodermal 

 cells at the blind ends of the stomodaeal and proctodaeal invaginations. 

 These masses become cup-shaped, the edges of the anterior growing 

 toward the edges of the posterior cup until their edges meet, enclosing a 

 part of the mid-gut yolk. The yolk, previous to this period, has become 

 enclosed first by the provisional dorsal closure soon after blastokinesis and 



stom 



eci— . 



A B C 



Fig. 166. — Locusta. Sagittal sections. A, stomodaeum of 52-hour embryo. 5, 

 stomodaeum of 56-hour embryo. C, proctodaeum of 70-hour embryo, {am) Amnion. 

 (ec<) Ectoderm, {lb) Labium. {Ir) Labrum. {md) Mandible, (mes) Mesoderm, {mx) 

 Maxilla, {prod) Proctodaeum. {stom) Stomodaeum. 



then by the splanchnic mesoderm which grows around the yolk outside 

 the provisional dorsal closure, thus cutting off the little that remains of 

 the extraembryonic yolk. The provisional dorsal closure then degener- 

 ates, leaving the thin splanchnic mesoderm as the second provisional 

 mid-gut cover until four days after blastokinesis. One day later (two 

 days before hatching) the cell masses at the blind ends of the stomo- 

 daeum and proctodaeum quickly spread over the entire yolk to form the 

 definitive mid-gut epithelium. At the same time, the stomodaeal and 

 proctodaeal membranes rupture, thus connecting the lumen of the mid- 

 gut with those of the fore- and hind-guts. 



In the 56-hour-old embryo the dorsal walls of the mandibular coelom 

 (Fig. 153) give out large, rounded, loosely arranged cells which spread 

 mediodorsally and soon form an arched structure, the subesophageal body. 

 It ultimately comes to lie beneath the stomodaeum, losing its connections 

 with the coelomic walls. The cells of the subesophageal body reach a 



