62 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



Segmentation becomes externally evident during the latter part of 

 the third day of development. The early distinguishable procephalic 

 lobes form three segments : the labral, the antennal, and the inter- 

 calary. The protocormic region gives rise to 17 segments. The first 

 three are the gnathal segments and the next three are those belonging 

 to the thorax. The remaining 11 form the abdomen. 



The digestive tract arises partly from anterior and posterior invagi- 

 nations of the ectoderm. These are the stomodaeum and proctodaeum 

 respectively. They grow toward each other pushing the mesenteron 

 rudiments before them. The proctodaeum arises in conjunction with 

 the amniotic cavity. The stomodaeal invagination produces the fore- 

 intestine and the proctodaeal invagination gives rise to the hind- 

 intestine. The anterior and posterior mesenteron rudiments each 

 proliferate to form two lateroventral ribbons. The mesenteron ribbons 

 of each end grow toward and ultimately fuse with those of the opposite 

 end. They also widen until they completely enclose the remaining 

 yolk mass, thus forming the epithelium of the midintestine. The 

 ventral completion of this lining is accomplished first, dorsal closure 

 being delayed until after the absorption of both dorsal organs. The 

 four malpighian tubules develop as diverticula of the proctodaeum. 

 They are evident by the end of the third day of development. 



The central nervous system arises from two lateral cords, one 

 below each neural ridge, and from a middle cord which is located 

 below the neural groove. Two pairs of ganglia, the protocerebral and 

 deutocerebral, form anterior to the stomodaeum. A third pair, the 

 tritocerebral, originating posterior to this invagination, moves forward 

 and fuses with the first two pairs to form the definitive brain or supra- 

 oesophageal ganglion. The three pairs of ganglia which originate in 

 the gnathal segments coalesce to produce the suboesophageal ganglion. 

 The three thoracic ganglia remain distinct. Of the 10 abdominal 

 ganglia, the last 3 fuse to form one definitive ganglion. The nerve 

 cord shortens in the late embryo so that not all of the abdominal 

 ganglia lie in the segments of their origin. 



Sixteen pairs of coelomic sacs are formed. They arise in the intra- 

 segmental portions of the mesodermal bands, one pair in the deuto- 

 cerebral segment and in each of the first 15 segments posterior to the 

 stomodaeal invagination. These sacs are thick-walled and possess 

 small lumina. They soon break open and join their cavities to the 

 epineural sinus to form the definitive body cavity or haemocoel. 



The gonads are prominent embryonic organs. They are produced by 

 the formation of a splanchnic mesodermal sheath about the gonial 



