324 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



tion (Figs. 277, 278). Along the dorsal mid-line is a strip of cells differing 

 from the remainder in being especially thin and flat and in maintaining a 

 close relation with the yolk (Fig. 277). 



The mesoderm is formed from a median longitudinal area of the ven- 

 tral blastoderm, the middle plate (Figs. 279, 280, mp) which separates 

 from the blastoderm on each side of it. These lateral parts constitute 

 the lateral plates (Ip). The middle plate sinks below the surface (Fig. 

 281 A) while the lateral plates approach each other and finally are united 



Fig. 280.— Apis. 



Cross section, (dr) Dorsal strip. (Ip) Ectodermal lateral plate, (mp) 

 Mesodermal middle plate, (ser) Serosal cells. 



along the ventral mid-line to form the ectoderm (ect). The rudiments 

 of the mid-gut epithelium are formed by the inward immigration of 

 blastoderm cells, a discoid swelling being thus produced at each of the 

 two ends of the middle plate but outside its hmits. These rudiments 

 later are covered by the ectoderm, excepting that in the case of the 

 anterior rudiment a small circular area (mge. a) remains uncovered which 

 later constitutes the floor of the stomodaeal invagination (Fig. 282, mge). 

 During these changes the median dorsal area of the blastoderm, com- 

 posed of thin flat cells, also becomes depressed and is overgrown by the 

 dorsal margins of the lateral plates (Fig. 279). Meanwhile the cells of 



