loo The Life-History of Insects 



the air-tubes, by inpushings of the ectoderm, which 

 meet and unite with the internal, mesodermal ducts. 



Polar Cells. — In some Flies and Midges the rudi- 

 ments of the germ-cells have been traced yet further 

 back. While the primitive cell-segmentation is still 

 in progress, before the formation of the inner layer, a 

 " polar cell"^ is separated from the hinder pole of the 

 egg. This by successive halving divides into eight 

 cells, which, after the formation of the blastoderm, 

 pass through it, and divide into two groups, taking 

 up positions on either side of the hind-gut. From 

 these cells the eggs or sperm-cells of the developing 

 insect arise (65)- This very early separation of the 

 primitive germ-cells is of great interest, as it lends 

 support to the well-known theory of Weismann (57) 

 that, in all animals, the nuclear matter (germ-plasm), 

 which passes on the inherited characters of the race 

 from generation to generation, is distinct from the 

 nuclear matter whose division gives rise to the body 

 of the individual. It is thought by those who hold 

 this view that in all cases the primitive germ-cells 

 are distinct from the ordinary cells of the embryo, 

 though only a few instances are known in which this 

 distinction is clearly marked and capable of being 

 observed. 



Hatching. — The time occupied by development 

 within the egg varies from less than a day in the 

 Blow-fly to nine months in many insects. Locusts, 

 Moths, etc., — whose eggs, laid in summer, do not 

 hatch out till the next spring. Some insects, after 

 hatching, undergo a great change in form. The 

 embryo Cockroach, for instance, is three times as deep 

 as wide, but soon after its escape from the egg- 

 membrane these proportions become reversed, and the 



1 It is important that these cells should not be confused with the 

 polar bodies or directive corpuscles described above (p. 86). 



