330 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



mge-^'" 



the yolk. The caudal extension of the anterior rudiment and the 

 cephalic extension of the posterior rudiment (Fig. 285, mge) next meet on 

 the dorsal surface of the yolk about one-third the length of the egg from 



its cephalic pole. The epithelial 

 strip thus formed extends rapidly 

 ventrad over the sides of the yolk 

 until the latter is completely 

 enclosed, the two margins of the 

 epithelium meeting and uniting 

 along the ventral mid-line of the 

 yolk a short time before hatching 

 (Fig. 2SlC,mge). Both fore- and 

 hind-guts are formed as usual by 

 ectodermal invaginations (Fig. 285) . 

 The stomodaeal invagination is, 

 however, not completely ectoder- 

 mal, since its floor is formed by cells 

 belonging to the anterior mesenteron 

 rudiment, which is not covered by 

 ectoderm. The hind-gut is exclu- 

 sively ectodermal. The lumen of 

 the stomodaeum becomes connected 

 with that of the mid-gut shortly 

 before hatching. A proventricular 

 valve is also formed at this time by 

 folding of the stomodaeal or eso- 

 phageal wall. The lumen of the 

 proctodaeum is at no time in con- 

 nection with that of the mid-gut, 

 both the cephalic end of the hind- 

 gut and the caudal end of the 

 mid-gut being blind. The four 

 Malpighian tubules are formed as 

 ectodermal invaginations which 

 make their appearance prior to the 

 formation of the proctodaeum, 

 grouped around the point where the 

 proctodaeum is to appear. Four 

 separate invaginations have not 

 been observed, the pair situated on each side of the mid-line being con- 

 nected by a shallow crescentric groove. 



The primary yolk cells, which are derived from cleavage cells remain- 

 ing within the yolk, multiply by. mitosis, the mitotic figures being at first 



mge- j a^ 





Fig. 285. — A-pis. Sagittal sections. A, 

 anterior, B, posterior ends, (m^e) Mid-gut 

 rudiments, (proct) Proctodaeum. 

 Stomodaeum. 



