SIPHON APTERA AND DIPT ERA 



373 



width (Figs. 328A,B). The anterior piece is again divided by a curved 

 furrow into two parts. Laterally the five ventral lines are bounded by a 

 longitudinal line on each side. These lateral lines mark the edges of the 

 lower layer, or middle plate. 



The five lines are not furrows but represent differences in thickness of 

 the inner protoplasmic layer showing through. Later, as development 

 progresses, they become furrows. The germ band continues around the 



A B C D 



Fig. 328. — Calliphora erythrocephala. A, lateral; B, ventral; C, lateral ventral; D, 

 dorsal aspect. Successive stages. A, formation of germ band. B, development of 

 median plate. C, closure of mesodermal tube. D, postmesenteron rudiment invaginated 

 into the yolk, {ent) Entoderm, (/r) Furrow, {gb) Germ band; (a) anterior; (w.) median; 

 (p) posterior, {gc) Germ cells. 



egg at both poles and is not restricted to the mid-ventral section, as 

 Voeltzkow believed. The middle section of the germ band now begins 

 to sink below the level of the surface, and the cells of the band become 

 quite deep. The middle section of the band is sharply limited posteriorly 

 by the posterior furrow, anteriorly by the converging fold. From this it 

 is seen that the entire band is composed of an anterior, a middle, and an 

 end piece. The middle section now forms a depressed area from which 

 the mesoderm is formed ; from the anterior and posterior parts the mid-gut 

 epithelial rudiments are to develop. It was formerly believed that the 

 middle section of the germ band between the anterior and posterior 

 transverse furrows constituted the entire lower layer. However, as has 



