238 EMBRYOLOGY [CH. 



the mandibles and first maxillae. As the appendages grow, they 

 become directed backwards. 



Posterior to the third legs, the segmentation of the body 

 continues backwards. Rudimentary pairs of appendages are said 

 to be discernible for a short time on all the abdominal segments ; 

 but they soon disappear, except on the eleventh segment, where 

 they persist as the cerci. 



This stage in the growth of the embryo is completed by its 

 stretching or straightening out, so that the head is drawn 

 inwards towards the posterior pole of the egg, and comes to 

 lie in line with the rest of the body (G). At this stage it will 

 be noticed that the cephalic lobes have extended back more 

 dorsally, while they have also fused ventrally with the median 

 front of the head, which now is seen to form a kind of flattened 

 lobe, destined to become the labrum and clypeus. Thus the head is 

 much more completely formed than it was before the straightening 

 process set in. It should be noticed that the amnion and serosa 

 are still fused at a point near the original position of the blastopore, 

 and that the embryo now lies longitudinally within the egg, with 

 its head downwards and close to the posterior pole. 



Rupture of the Amnion, and Reversion of the Embryo (fig. 109 HK). 



The embryo now undergoes a remarkable series of changes in 

 position, during which great alterations take place in the amnion 

 and serosa. The latter appears to thicken and contract, drawing 

 all the yolk left near the posterior pole of the egg forwards towards 

 the anterior pole. The increase of pressure consequent upon this 

 movement, and upon the steady growth of the embryo, causes 

 the amnion to rupture along the portion above the head, where it 

 is fused with the serosa. The amnion here splits into two halves. 

 As the serosa goes on contracting, the pressure causes the embryo 

 to project head first through the split (H). The head of the embryo, 

 once freed from the amniotic envelope, turns round and follows 

 the contracting serosa upwards, towards the anterior pole. The 

 rest of the body of the embryo follows the head in due course, 

 so that the embryo is for a time curved round on itself, while 

 segment after segment passes out through the rent (H, j). Finally, 

 the head comes to lie below the anterior pole of the egg, while the 



