242 EMBRYOLOGY [CH. XH 



4. The haemocoele is a secondary enlargement of the blood 

 lacunae, formed in the mesoderm. This increases enormously, 

 and forms the general body-cavity of the insect. 



5. The fat-body is formed by proliferation of the cells lining 

 the haemocoele. 



6. The muscles and connective tissues are formed from the 

 somatic layer, except the muscles of the alimentary canal, which 

 are derived from the splanchnic layer. 



7. The gonads and their ducts are derived from two longitudinal 

 strands of cells formed in the somatic mesoderm. These genital 

 cells, from which the gonads arise, are formed very early in 

 embryonic life, as specialized mesoderm cells lying towards the 

 posterior end of the ventral plate. Later, they spread out from 

 the seventh to the second abdominal segments. Undiflerentiated 

 mesoderm cells form an epithelium surrounding the true genital 

 cells. 



C. Parts of Endodermal Origin. 



The only part formed in the Dragonfly from the endoderm is 

 the epithelium of the mid-gut, excepting the short anterior and 

 posterior pieces already mentioned as being of ectodermal origin. 

 As already stated, even this is doubted by Heymons. 



Length of Embryonic Life. 



The time elapsing between the laying of the egg and the 

 hatching of the larva is about three weeks in the case of the 

 Zygoptera, a little less in Aeschnidae. But some Libellulidae, at 

 any rate, if not all, hatch out much sooner. Diplacodes haematodes 

 hatches out five days after the eggs are laid, and some other 

 species almost as quickly. 



