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distinct though partial differentiation; the upper epidermis is very well 
defined, being composed of large cuboidal cells (fig. 3, at right); the meso- 
phyll shows three distinct layers, and the lower epidermis is set off from the 
other layers by the minute size of the units composing it. The chloroplasts 
of the mesophyll cells are well developed as to size and number. 
A number of modifications occur coincidently following the insertion 
of the setal proboscis. These are: (i) The reaction of the cytoplasm of the 
cells penetrated by the setal structure to this foreign structure, by laying 
down around it a deposit of organic substance in the form of a very thin, 
uniform membrane. This structure thus constitutes a definite sheath. 
(2) A marked hypertrophy of the lower epidermal cells and in a lesser degree 
of the adjacent mesophyll cells. (3) Hyperplasia sets in in the middle region 
of the mesophyll surrounding the end of the proboscis. This, however, is 
greatly restricted. In the case of the P. mamma gall (fig. 3), wall formation 
accompanies the nuclear divisions almost invariably in the early stages; 
while in the case of P. asteriscus wall construction does not occur, the 
original cells of the mesophyll tiers thus being left almost undisturbed, the 
cells, however, containing many nuclei. This is best demonstrated in a 
somewhat later stage (fig. 5&). (4) Partial degeneration of the chloroplasts 
of cells beneath the insect, involving loss of chlorophyll and more or less 
reduction in size. (5) An increase in the size of the nuclei as compared 
with those of the normal parts. 
The total result of the early hypertrophic and hyperplastic changes is 
the formation of a saucer-shaped depression in which the nymph (not shown 
in section) passively lies (fig. 3). This depression or evagination is produced 
chiefly through the hypertrophy of the elements on the under side of the 
leaf or that opposite to the insect. 
Figure 4 presents a median section of a later stage of the P. mamma gall. 
It will be noted that, compared to the tissue lying at some distance from 
the larva, the tissue immediately beneath the insect and adjacent to the 
proboscis shows a marked inhibition of cell-divisional activity. This 
condition is very characteristic of both galls. These non-dividing cells, 
however, show the same high protoplasmic content that the actively dividing 
cells do, so that, when the section is viewed as a whole, there appears to be a 
zone of meristematic tissue traversing the young gall (shaded cells, fig. 4). 
The whole situation, despite the fact of the local growth inhibition 
mentioned, when contrasted with the condition in a normal partially dif- 
ferentiated leaf, shows that in the early stages of cecidogenesis a process of 
dedifferentiation is going on, throwing the tissue back into a homogeneous 
condition. Were events to stop here (fig. 4), we should have a typical 
kataplasma showing the retrogressive changes of that type of overgrowth. 
The gall in its further development, however, differentiates into a highly 
specific prosoplasma, thus furnishing a characteristic example of gall 
ontogeny recapitulating gall phylogeny. 
