XII] 



EMBRYOLOGY 



239 



rest of the embryo is in line with it, running back to near the 

 posterior pole, where the bent up end of the abdomen lies. These 

 movements of the embryo are termed blastokinesis. All the yolk 

 is now contracted into the space dorsally above the embryo, 

 which lies with all its appendages facing outwards along the 

 ventral surface of the egg. 



During the above changes, the remains of the amnion have 

 gone on contracting with the serosa, until finally the two mem- 

 branes become sufficiently reduced in area to form a tight covering 

 for the dorsal yolk-plug 1 . Thus, at stage K, we may see how 

 the embryo is really only formed laterally and ventrally, while 

 the whole of its dorsal portion consists of this huge yolk-sac. 

 By a gradual extension of the ectoderm and mesoderm laterally, 

 and then latero-dorsally, over the yolk- 

 sac, the latter comes at last to lie com- 

 pletely within the body of the embryo. 

 The final closure takes place along the 

 mid-dorsal line. 



Further Changes. 



The embryo now grows steadily, so 

 that the head comes closer and closer up 

 to the anterior pole of the egg. The 

 antennae, second maxillae and the three 

 pairs of legs are soon seen to be dis- 

 tinctly jointed. The segmentation of 

 the whole body becomes distinct and 

 annular. Reddish pigment areas on 

 either side of the head indicate the first 

 formation of the compound eyes, which 

 soon appear blackish in the middle. 

 The yolk-plug steadily contracts, and 

 finally becomes all used up, except for 

 the portion enclosed in the mid-gut. 

 The mandibular and maxillary teeth appear, and the bases of 



Fig. 112. Anterior half of 

 egg of Anax papuensis 

 Burm., just before hatch- 

 ing, ventral view (x60), 

 ant antenna; cl clypeus, 

 e compound eye; fbm la- 

 bium ; Jbr labrum ; md man- 

 dible; mx first maxilla; 

 Pi~Pa ^ G gs. Original. 



1 The actual fates of the amnion and serosa are doubtful. It is believed that 

 the serosa gradually breaks up and mingles with the yolk, while the final fate of 

 the amnion is not definitely known. 



