230 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



anf 



-lb 



J/ 



Coelom formation in the first abdominal segment takes place in the 

 same way as in the head and thorax. In 

 the following segments with the grow^th of the 

 median portion of the ectoderm the originally 

 wedge-shaped mass of the inner layer flattens 

 out and spreads laterally in such a way as to 

 lose its multilayered condition and acquires 

 instead a bilayered arrangement of its nuclei 

 (Fig. 149). Soon the dorsal of these two 

 layers becomes extremely thin. Meanwhile 

 the undivided mass of the inner layer becomes 

 constricted into segmental masses, and then 

 between the two layers of cells a cavity appears 

 (Fig. 150). By subsequent division of each 

 segmental mass into a right and left haK the 

 paired coelomic sacs are formed. The medio- 

 ventral wall of the coelomic sacs in each 

 segment grows medially and thus secondarily 

 forms the mesoderm bridge from which the 

 blood cells arise (Fig. 151, mes). 



Between 56 and 59 hours a pair of labral 

 coelomic sacs with distinct cavities may be 

 distinguished. UnHke those of Carausius 

 (Wiesmann, 1926), they are not joined medi- 

 ally at this stage. By the 72-hour stage they 

 have disappeared, leaving in their place a 

 rather loose mass of mesoderm which still 

 shows a paired arrangement and which soon 

 afterward becomes connected with the meso- 

 derm investing the stomodaeum and eventually 

 gives rise to the labral musculature. 



The antennary mesoderm is first differenti- 

 ated in the 52-hour stage as a pair of mesoderm 

 masses lying behind the stomodaeal invagina- 

 tion. Cavities develop in these masses 4 

 hours later and elongate with the growth of the 

 antennae. In the 72-hour stage each anten- 

 nary coelom has developed a long dorsorostral 

 pouch projecting far into the head and a much 



P-2 



p.3 



■ab.l 



© 



-eps 



•dc 



-ab. 



■ proct 



Fig. 152. — Locusta. Para-sagittal section of 75-80-hour embryo. Coelomic sacs, {ab 

 1-11) Coelomic cavities of abdominal segments 1 to 11. (ant) Antennal, (lb) labial, (md) 

 mandibular, (mx) maxillary, and (p 1-3) thoracic coelomic cavities, (am) Amnion, (br) 

 Brain, (dc) Provisional dorsal closure, (eps) Epineural sinus, (int) Intercalary append- 

 age, iproct) Proctodaeum. (suboesb) Subesophageal body. 



