Packard.] INSECTS OF THE PLANT HOUSE. 
107 
become filled with } 7 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 pressed 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 yolk cells. Finally feet and 
jaws and antennae bud out from the band, until the 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 antennae and jaws and the 
legs are in their position. Identical in form, the antennae 
and jaws differ from the limbs 
simply 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 w*e 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 antennae, 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 clearly demonstrated by Metznikoff 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, antennae; 2, mandibles; 3, first pair of maxillae; 4, second pair; 5-7, legs. 
11 
Fig. 73. 
