DEVELOPMENT OF CERTAIN PACHYPSYLLA GALLS 
281 
mite gall on the leaf of the European linden, Kiister found, in addition, 
the degeneration of one nucleus to be a constant phenomenon. 
The multinucleate cells found in this study are of much interest, not 
only because such cells have been rarely reported in the higher galls, but 
also because a special situation was found in which many of the nuclei 
were undergoing a process of disintegration. 
In the P. asteriscus gall, by the time the gall has attained a length of 
I mm., many of the cells, both the one in which the stylets end and certain 
of those in the immediate neighborhood, show more than one nucleus 
(fig. 56). The median cell holding the opening of the sheath-enclosed 
proboscis has generally the largest number, as many as four to six being 
visible in one focal plane. The other cells possess fewer, more than two 
being seldom found definitely showing in the same focal plane. Only a 
few cells in such a section as shown in figure $h are thus seen (oil immersion 
lens), but these are unmistakably multinucleate. 
The nuclei within a particular cell upon critical examination do not 
appear to be all in the same condition. One commonly, or at the most 
two of them, may be regarded as normal, while the others exhibit varying 
stages of disintegration (fig. 5c, scale same as fig. 9a). In this disintegration 
process the nuclear membrane disappears, and a prominent vacuole de- 
velops in place of a part or all of the nuclear body. The nucleolus persists 
quite unchanged and may often be found stranded in the cytoplasm near a 
vacuole. 
In the I mm. gall of P. mamma practically no multinucleate cells are 
found aside from the large central one in which the mouth parts terminate. 
But in much older material (the half-grown gall) excellent examples of the 
giant-cell condition are found. In these, as in the cells of the P. asteriscus 
gall, all stages of disintegration of the nuclei excepting one or two are 
plainly evident (fig. 9a). The problem of the mode of origin of these 
nuclei is an important one since in some instances, such as the giant cells of 
the nematode (Heterodera) galls, nuclear proliferation has been reported 
as amitotic in character. After a most painstaking examination of my 
slides I am strongly inclined to interpret the situation in terms of amitosis. 
I am not able, however, to furnish direct positive proof. 
The total absence of mitotic figures from the cells of the region con- 
cerned might be due merely to the fact that this region is characterized by a 
marked inhibition of growth as compared to adjacent regions. Individual 
cells having their origin in the normal embryo leaf before the insect's 
attack, later, under the inhibiting influence exercised on the three cell 
layers beneath the nymph, retain their integrity even into the adult gall 
stage, changing only in the matter of numerical nuclear increase. 
This latter change is then followed by degeneration of certain of the 
nuclei. Since the number of nuclei per cell is always relatively low, seldom 
if ever going over eight, and since the divisions producing these are scattered 
