124 INJURIOUS AND USEFUL INSECTS 



becomes infolded and thins away at one place, so as to allow 

 the yolk to reach the surface. The body has henceforth two 

 ends, a head-end and a tail-end, though they are at first not 

 very unlike each other. The embryo is, during this and the 

 next three stages, curled round the yolk, with what will 

 ultimately be its ventral surface outwards. The cells of the 

 blastoderm divide rapidly along the under side, forming the 

 ventral plate, while the dorsal part of the blastoderm thins 

 out. The cavity is still occupied by granular yolk. About 

 this time the sexual germ-cells re-enter the egg, apparently by 

 forcing a passage through the blastoderm. The ventral plate 

 at first lay along the convex side of the egg, while the future 

 head and tail were on the fiat side. Now the embryo rotates 

 on its principal axis, so that the ventral plate lies along the 

 flat side of the egg. The rotation occupies about a quarter 

 of an hour. The first day of development is thus completed. 



Stage 4. Early in the second day the ventral plate be- 

 comes transversely constricted. The segments which form 

 first are those which enter into the future head. Others are 

 rapidly added behind these. The body contracts, and the 

 head and tail become more and more widely separated. 

 Towards the middle of the second day the embryo rotates 

 a second time on its axis, and the ventral plate comes to 

 lie once more on the convex side of the egg. 



Stage 5. The body is now segmented throughout. Sixteen 

 segments can be counted, and some of these are beginning 

 to push out paired appendages, the rudiments of jaws or legs. 

 The body has now contracted and straightened itself to such 

 an extent that the head is at one pole of the egg and the tail 

 at the other. The nerve-cord has made great progress. Its 

 first rudiment appeared on the ventral side of the body, 

 towards the fore end, and rapidly grew till a gangliated chain, 

 extending the whole length of the body, was formed. Owing 

 to its transparency, the nerve - cord can hardly be made 

 out till a later stage unless stained sections are employed. 

 At first the ganglia are relatively large, and occupy almost 

 the whole cord. 



Stage 6. A pit is formed by pushing the skin of the under 

 side of the head inwards. A second pit forms by the pushing 

 of the skin inwards at the tail-end. These two pits lengthen, 

 and ultimately form the fore-gut (gullet, etc.) and hind-gut 



