No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 19 



in the yolk with its cephaHc amnion apphed to the ventral 

 serosa and the amnion overlying its terminal abdominal seg- 

 ments applied to the serosa covering the dorsal yolk. No 

 sooner has the caudal end become fixed than the head is 

 detached from the ventral face of the o.^^ and the embryo 

 swings back, straightens out, and becomes applied full length 

 to the dorsal serosa. The movements whereby this condition 

 is attained resemble the movements of a leech in passing from 

 one side of a test-tube to the opposite surface; holding fast to 

 the glass by means of the oral sucker, the tail is stretched out 

 till it reaches the opposite surface, when the anterior end is 

 loosened and the body drawn over. There is, however, a 

 marked difference between the embryo and the leech since 

 the body of the former is not contracted during its transition. 



Fig. 5 represents a rather rare condition in that the pro- 

 cephalic lobes lie at the same level and are symmetrically dis- 

 posed with respect to the long axis of the egg. More frequently 

 the germ-band is twisted during its transition so that one of the 

 procephalic lobes reaches further forward than the other on 

 the surface of the yolk. Sometimes it is the left lobe which 

 extends further forward but more frequently it is the right. 

 The twist in the germ-band occurs in the thoracic or 

 abdominal region, more often in the former, the abdomen being 

 nearly straight. I take this twisting of the embryonic axis to 

 indicate that the germ-band executes a screw-like movement 

 while penetrating the yolk, and I believe it to be perfectly 

 normal, having observed it in the majority of embryos. Traces 

 of this twisting are clearly discernible even in embryos which 

 have almost straightened on the dorsal surface. 



As a consequence of the passage of the embryo through the 

 yolk in the manner above described, the germ-band has shifted 

 its position from the median convex ventral to the median 

 concave dorsal surface of the yolk, so that it is now reversed : 

 originally its head pointed to the tapering anterior pole, now 

 it lies with its head directed towards the blunt posterior pole 

 of the tg^. The amnion, of course, accompanies and remains 

 in close contact with the ventral surface of the embryo during 

 all this time. 



