1907] GATES—POLLEN DEVELOPMENT AND MUTATION 105 
: spirem threads in synapsis, as a preliminary to sporogenesis; then, 
f as these authors point out, the synapsis stage becomes the critical 
point in the life cycle of the individual. It is conceivable that the 
maternal and paternal chromatin after entering the same nucleus 
in fertilization could continue in this condition throughout the sporo- 
phyte history without interfering with a development, but that the 
intimate union which occurs in synapsis, where an interchange of 
material between the parental idioplasms probably takes place, 
might lead to the development of “incompatibilities” between the 
plasms, such as are exhibited in irregularities and disturbances in 
the reduction divisions, and finally more or less complete failure of 
later development. It may be remembered in this connection that in 
O. lata no irregularities in the prophases were observed until after the 
contracted condition of the spirem. 
This discussion is intended to serve merely as a partial analysis 
of the problem, of which it does not offer a solution. A larger body 
of facts is very desirable, correlating cytological studies of hybrids 
with observations of their variations and their relations to the parental 
forms in all their characters. It may then be possible to determine 
whether observable differences in the mechanism of inheritance as 
exhibited by the chromosomes occur in Mendelian, intermediate, 
variable, and other types of hybrids. 
THE CHROMOSOMES AND MUTATION 
It is believed that one of the most important discoveries connected 
with the investigation thus far has been the observation of certain 
chromatic bodies with a peculiar origin and history, which have 
been designated heterochromosomes. Since they have been found 
in the dominant Lamarckiana xX lata hybrid, I believe it is probable 
that they will also appear in O. Lamarckiana itself. This point is to 
be investigated as soon as possible. If this surmise is found to be 
true, then the pollen mother cells in which these bodies occur must 
differ in chromatin content, and hence probably in hereditary value, 
from those in which they are absent, provided that pollen grains from 
both these classes of mother cells reach maturity. It is highly prob- 
able that the latter supposition is true in the O. Lamarckiana hybrid, 
for an abundant amount of pollen reaches maturity, so that in crossing 
