NEUROPTERA AND COLEOPTERA 293 



with the tail (Fig. 231). By this time the brain (hr) and the ventral nerve 

 cord are well differentiated. The shortening of the body with the con- 

 sequent shrinking and thickening of the dorsal amniotic membrane gives 

 rise to the so called "dorsal organ" which now forms a pear-shaped mass 

 lying in the neck region (Fig. 231, do), its anterior end still attenuated. 

 Soon the cells bordering the dorsal organ by proliferation form first the 

 definitive closure of the dorsal wall posteriorly and laterally and finally 

 anteriorly, the dorsal organ becoming smaller and smaller until it finally 

 disappears. 



The cells of the proctodaeum at the time the dorsal organ is still 

 present (Fig. 231) are large, with well-marked nuclei, but Noskiewicz and 



stom 



^proct 



mes do 



Fig. 231.^ — Stylops gwynanae. {hr) Brain, (do) Dorsal organ, (ffc) Germ cells, {i. 

 am) Inner section of amnion, {mes) Mesoderm, {neurp) Neuropile of subesophageal 

 ganglion, {prod) Proctodaeum. {stom) Stomodaeum. {y) Yolk. 



Poluszynski observed no proliferation of cells at the extremity that could 

 be interpreted as mid-gut epithelium. As development continues, the 

 proctodaeal cells diminish in size and finally resemble adjacent cells. At 

 the inner end of the stomodaeum at this time proliferating cells are seen 

 (Fig. 231, stom) which as time goes on push in under the yolk as well as 

 over it until only a small section just below the dorsal organ remains 

 uncovered. Shortly afterward the wall separating stomodaeum and 

 mid-gut ruptures, thus establishing continuity. No evidence was found 

 by these investigators to warrant the assumption that the mid-gut was 

 formed either from entoderm rudiments located at the tips of the stomo- 

 daeal or proctodaeal invaginations or from cells from a middle strand of 

 the inner layer, though the latter possibility was not denied. Be that as 

 it may, the fact remains that the mid-gut epithelium arises from cells 



