414 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



although opinions on this are divided. Pflugf elder (1932) failed to 

 demonstrate it in Platyrhacus. 



The first three rump segments, which correspond to the thorax of 

 insects, each bear but a single pair of legs; in later (postembryonic) 

 development the remaining rump segments have two pairs each. In 

 Platyrhacus, however, Pflugfelder has shown that two pairs of coelomic 

 sacs and two ganglia are formed in each of the first three segments, 

 indicating their double nature despite their single pair of legs. The 

 double segmentation may be a secondary adaptation. 



During postembryonic development an invagination appears on each 

 side of the fifth rump segment of the six-legged larva where the repug- 

 natorial glands are to appear. Later, with the increase in number of 

 segments, additional glands will arise, but a single pair only for each 

 segment. They are lacking on the "thoracic segments." 



The singular formation of the mid-gut epithehum as a strand of cells 

 passing through, not surrounding, the yolk is a characteristic of the 

 Diplopoda noted by both the earlier and later writers. The yolk lies 

 in and fills the body cavity, so that the nervous system as well as the 

 alimentary canal is surrounded by yolk. 



The young animal on emerging from the chorion is a helpless six- 

 legged creature, and only after another molt does it become active. In 

 rare cases, as in Polygonium germanicum,, there is present a fourth pair of 

 legs, belonging to the first abdominal segment. In general, one or more 

 pairs of legs are added with successive molts during adolescence. 



SYMPHYLA, THE SYMPHYLIDS 



Hanseniella sp. 



Tiegs (1939) has found that in Hanseniella, a genus of the class 

 Symphyla, the cleavage is total, as in CoUembola and in some diplopods. 

 By tangential division of the large yolk pyramids, an outer layer of small 

 cells and an inner layer of very large cells arise, both rich in yolk. From 

 the former the blastoderm develops. From the latter arises by further 

 division the internal mass of yolk cells. These are not pure vitellophags, 

 however, for from them the fat-body and the mid-gut epithelium develop. 



An embryonic area forms by ventral thickening of the blastoderm. 

 A precocious flexure then forms in the embryonic area, as in some diplo- 

 pods, and thereby the germ band becomes defined. A dorsal organ is 

 present, but embryonic membranes are lacking. The mesoderm arises 

 by bilateral separation of cells from the germ band. Segmentation of 

 the mesoderm into somites synchronizes roughly with external segmenta- 

 tion of the germ band. In the head there is no external sign of seg- 

 mentation. Behind the labium are formed seven leg-bearing segments, 



