240 EMBRYOLOGY [CH. 



the three- jointed second maxillae fuse in the middle line, 

 indicating the first formation of the labium. A pale brownish 

 cuticle can be seen enclosing the whole embryo. 



Just before hatching, the position of the cephalic and thoracic 

 appendages is as shewn in fig. 112. It will be seen that the 

 original front lobe of the head is now divided into clypeus and 

 labrum. The antennae are two- jointed and slender. The mandibles 

 and first maxillae point rather inwards, and shew their definite 

 shapes. The second maxillae lie parallel and backwards, re- 

 sembling a pair of legs. From this point the reader may continue 

 the narrative by turning to the account of the hatching of the 

 larva on p. 67. 



Origin of the Parts of the Embryo. 



A. Parts of Ectodermal Origin. 



1. The hypoderm is derived directly from the ectoderm, which 

 comes finally to enclose the whole body of the embryo as the 

 layer of hypoderm cells. 



2. The cuticle is secreted by the hypoderm. Two cuticles are 

 formed, one inside the other, during late embryonic life. The 

 outer one forms the pronymphal sheath (p. 69). 



3. The tentorium is developed from two pairs of ectodermal 

 invaginations in the forepart of the head. The details have not 

 been worked out in the Odonata. 



4. The nervous system. On either side of the mid-ventral 

 line of the body of the embryo, the ectoderm becomes thickened 

 into a longitudinal cord. This pair of cords sinks below the rest 

 of the ectoderm, which grows ventrally over them. The cords then 

 become segmented, and give rise to the ventral chain. The two 

 cords fuse in each segment to form the paired segmental ganglia, 

 from which the longitudinal nerve-cords grow out separately later. 

 The development of the brain and oesophageal commissures has 

 not been worked out in the Odonata. 



5. The compound eyes. These organs become fairly well 

 developed during embryonic life. Their early formation, however, 

 has not been studied in the Odonata, so far as I am aware. The 

 ocelli do not develop at all in the embryo. 



