Packaud.] insects of THE TLANT HOUSE. 



107 



become filled with 3'olk cells or granules. After fertilization 

 the nucleus subdivides into smaller cells. These seek the 

 outer region of the egg. They multiply a thousand fold, 

 become i)ressed together, lose their character as distinctive 

 cells and form a pale band partly or wholly surrounding the 

 yolk mass. This is the primitive band, the germ, which 

 grows at the expense of the j'olk cells. Finally feet and 

 jaws and antenna bud out from the band, until tiie form of 

 an Aphis is rudely sketched out. Mark the fact, one of the 

 most interesting in the morphology of animals, that at first 

 the only difference between the antennaB and jaws and the 

 legs are in their position. Identical in form, the antennie 

 and jaws differ from the limbs fig. 73. 



simpl}' in the fact that they are 

 situated in front of the legs. 

 Now as each pair of appendages, 

 whether legs or jaws or antennae, 

 indicate a segment or ring, we at 

 once get a clew by which we can 

 easily settle the question of the 

 number of segments in the head 

 of the winged insects. Here, 

 following one another in orderly succession, are four pairs 

 of protrusions like the fingers of a glove, beginning with 

 the antenune, the foremost, and ending with the labium, the 

 pair next the legs. So we have four segments in the head. 

 In after life the segments, clearly to be seen in the embryo, 

 become so coalesced that it is impossible to define their 

 limits. In most books the head is quite wrongly counted as 

 one segment. 



An important point clearl}' demonstrated by MetznikofiT is 

 one bearing on the question of the origin of sex. It is 

 maintained by some that well-fed caterpillars, for example, 

 produce female butterflies, while starved ones produce males. 



♦ 1, antcnnw; 2, mandibles; 3, flrst pair of maxilla;; 4, aecoud pair; 5-7, legs. 



11 



12 3 4 5 6 7 

 Embryo of Dragon Fly.* 



