374 BOTANICAL GAZETTE [May 
once in the evolution or ontogeny of zoocecidia, how is such a 
retrogression followed by a progression in a new direction possible ? 
What is the mechanism involved? The effort of all students in 
attacking this so-called stimulus problem has signally failed to 
elucidate the situation. In the opinion of the writer this is because 
the problem goes much deeper than our present technique is able 
to penetrate, for it is to be classed with the general unsolved 
problems of growth (ontogeny) and evolution (phylogeny). Just 
as a mass of cells in an apical growing point, in some unknown 
manner, certainly has much to do with the differentiation products 
of that stem, so has the mass of embryonic cells constituting the 
prosoplasma-making larva much to do with the differentiation 
products, not only of itself but of the plant tissue around it. It 
has extended its control (mechanistically interpreted) in the field 
of form characters (and others to a less degree) beyond the borders 
of its own body. As Focxev (6) has put it: ‘La feuille est en 
rapport avec les phenomenes vitaux de la larve”’ rather than with 
the normal leaf itself (we would append to bring out the contrast). 
In other words, we would hold that the development of prosoplas- _ 
mas is brought about through»the superposition of embryonic 
animal tissue (the cecidozoon larva) ‘on that of embryonic plant 
tissue with a relation in growth which is an essentially normal 
one, that is, the mechanism of morphogenesis is operative, but the 
primary control-is with the larva. 
This leads to a final statement, one given earlier by the writer 
(12), to the effect that “the germ plasm of the cecidozoon is the 
place of origin of gall forms.” This, of course, merely falls in 
with the current general ideas concerning the significance of the 
germ plasm in evolution. In the germ plasm of the animal origi- 
nated the factorial conditions which, phylogenetically considered, 
first gave the embryonic cecidozoon the ability to break up the 
normal operation of the plant’s factors making possible normal 
plant differentiation, and secondly the factors which initiate the 
development of new form and other characters expressed in the 
plant cell masses. 
Zoocecidial evolution then is a complex in which, in its early 
stages (kataplasmas) with regard to certain characters, the plant’s | 
