The First Abdominal Segment of Embryo Insects. 
89 
pairs of rudimentary appendages on the abdominal segments have become 
visible. All the appendages are merely glove- finger-shaped outgrowths of 
the ectoderm into each of which extends a similar evagination of the un¬ 
derlying mesodermic somite. [Fig. 2.] Of the abdominal appendages all 
‘except the first pair soon disappear entirely. They are nipple shaped ele¬ 
vations which are brought down to the general surface of the abdominal 
ectoderm by the tension resulting from the lateral an 5 longitudinal growth 
of the embryo. The first pair of abdominal appendages in embryos twelve 
days old [Fig. 2 ap.], is twice as long as any of the succeeding evanes¬ 
cent appendages, and about half as long as the metatlioracic legs. 
Had I figured the whole of the sagittal section, of which Fig. 2 repre¬ 
sents only a small portion, it would be seen that the three pairs of thoracic 
appendages are of equal length, and together with the antennae, the 
longest and most prominent appendages of the embryo. The abdominal 
pair is longer than the mandibles in this stage, but only about two- 
thirds as long as either the first or second maxillae. Like the antennae, oral 
and ambulatory appendages, they are directed obliquely outward and back¬ 
ward. The evanescent abdominal appendages, on the other hand, are 
directed forward. 
The further differentiation of the antennae, mandibles, maxillae and feet 
is brought about by a very rapid proliferation of the constituent ectodermic 
cells, by a somewhat less rapid proliferation of the cells of the mesodermic 
somites of the appendages to form muscles, and by constrictions of the sur¬ 
face to form the joints of the adult insect. The differentiation of the first 
abdominal appendage is brought about in quite a different manner, 
namely, by the modification of the individual ectodermic cells of which its 
outer layer consists. 
The appendage ceases to grow much in length after the tvvefth day and 
its cells no longer divide. I have never seen a nucleus in any phase of 
either karvokinesis or karyostenosis, notwithstanding I have examined many 
sections in all stages of development. Hence the number of cells which 
constitute the appendage on the twelfth day remains constant till the or¬ 
gan disappears. The cells which form all but its basal portion increase 
enormously in size, assuming the shape of long prisms more or less attenu¬ 
ated in-some portion of their length. They grow inward and push the me¬ 
sodermic cells which at first grew forward into the appendage, back into 
the body of the embryo, The cyptoplasm of these large cells consists of 
finely and evenly granular protoplasm in which there are one or two spheri¬ 
cal or oval vacuoles at the peripheral ends. [Fig. 3, 4, u.] In surface 
views the cells are polygonal. The contour of the inner ends is indistinct 
in embryos fourteen days old. 
Not only does the cytoplasm of each of the formerly small ectodermic 
cells increase thus enormously, but also the nuclei, which assume a 
centrifugal position in the fanshaped sections of the appendage. Owing to 
their number the nuclei are forced to arrange themselves in several rows. 
