SIPHONAPTERA AND DIPT ERA 



379 



The rudiment by this time has become cleft at the free end into two 

 branches which are in contact with a ribbon of mesoderm that is destined 

 to form the muscle layers of the mid-gut. Later the mesenteron rudi- 

 ments elongate, and the stomodaeal invagination deepens (Fig. 332.4). 



A B 



Fig. 335. — Calliphora vomitoria. Section A is taken at the level of k of Fig. 329; B at the 

 level of u. (ect) Ectoderm, {ent) Entoderm, {mes) Mesoderm. 



Posteriorly at this time the posterior mesenteron rudiment is divided api- 

 cally into two branches in the same manner as the anterior rudiment. 

 With the shortening of the embryo the proctodaeal opening comes to lie 

 near the posterior end of the egg, with the result that the posterior mesen- 

 teron rudiment has its apex directed 

 anteriorly (Fig. 3325) . Subsequently 

 the elongation of the paired branches, 

 or ribbons, of the two rudiments meet 

 and then widen until their edges fuse, 

 forming the mid-gut epithelium, a tube 

 enclosing the yolk except for a small 

 portion of the yolk which remains in 

 the head region. The yolk cells take 

 no part in the formation of the mid- 

 gut epithelium. 



Escherich (1900) and Noack (1901) 

 both maintain that the anterior and 

 posterior mid-gut rudiments represent 

 the entoderm. Escherich, in common 

 with a number of other investigators, considered the longitudinal 

 ventral furrow as a much elongated gastrula furrow at the extremities 

 still preserving its original condition, the invaginated part formed exclu- 

 sively of entoderm (Fig. 335^, en^. At the anterior end but slightly 

 farther back lateral mesodermic diverticula appear (Fig. Sd5B,mes), and a 



mes 



Fig. 336. — Calliphora vomitoria. 

 Section taken slightly above the level 

 of X in Fig. 329. (ent) Entoderm. 

 (mes) Mesoderm. 



