94 GEORGO ORIHAY SHINJI 



end of the embryo has been situated at the posterior pole of the 

 egg because of its being the part first formed from the invaginated 

 portion of the germ band which occurred at or near the posterior 

 pole of the egg, but now it has accomplished its exchange of poles 

 by the process of revolution just mentioned. 



A side view of an embryo after its revolution has been com- 

 pleted is shown in figure 53. The thoracic limbs are now dis- 

 tinctly four jointed, showing the coxa, trochanter, femur, and 

 tibia. The segments of the limbs are almost equal in length, 

 but differ in shape, the second being the largest and the last 

 the most slender. The antennae also have four segments. The 

 most conspicuous feature at this stage is the appearance of the 

 deep invagination of the proctodeal opening on the dorsally 

 curled end of the abdominal segment. The proctodaeum invagi- 

 nates on the dorsal side of the ninth abdominal segment as will 

 become clear when its formation is studied in sections. Another 

 interesting feature of this stage is the sudden appearance of a 

 mass of cells on the dorsal side of the brain segment. In reality, 

 however, this mass of cells is formed during the revolution of 

 the embryo by the contraction of serosal and amnionic cells. 

 Another prominent change noticeable at this stage is that which 

 occurs in the formation of the mouth parts. 



The mandibles and the first maxillae in the stage last de- 

 scribed have migrated laterally and are no more in line with the 

 second maxillae. A large invagination between the second 

 maxillae and the first thoracic legs appears during the revolution 

 of the embryo around the shorter axis of the egg, and there 

 arises, from the pointed distal end of each of the mandibles 

 and the first maxillae, a slender chitinous bristle-like structure, 

 which is from the beginning non-cellular. The wall of the 

 invagination, on the contrary, is cellular at first, but later 

 becomes chitinous. The cavity thus formed and lined with a 

 chitinous layer, it must be mentioned, serves to accommodate 

 the bristles when they are not in use, as when the larvae changes 

 its feeding place. 



The upper lip, with a well-defined suture, does not elongate 

 or grow very much in size, but a layer of chitinous substance is 



