Mar. II, 1918 Sterility in the Strawberry 659 



(3) It is not until rapid growth of the microspores takes place and the 

 necessity of active cell metabolism appears that evidences of degenera- 

 tion appear. The necessity of active metabolism becomes apparent 

 when it is remembered that the microspores increase 7.5 times their 

 original volume during this growth period. 



(4) There is no specific time at which degeneration of the grains 

 within a single anther takes place. In most of the sterile forms thus far 

 studied a series of degenerating stages appear from the first period of 

 growth of the microspores to the formation of nearly mature pollen. 



It is becoming more and more evident that the growth and develop- 

 ment of plants and animals are directly dependent upon the chromosome 

 combination which they contain. Boveri {see Morgan, 28, p. 55), in 

 working with dispermic sea-urchin eggs, found that they very rarely 

 develop normal individuals, while if separated at the 4-celled stage 

 normal individuals often developed. This seems dependent upon the 

 fact that in the first four cells, which are the result of a single division, 

 the chances of one of the cells receiving at least one of each kind of 

 chromosome are relatively high and thus, when separated, some may 

 develop normally. On the other hand, the chances of each of the four 

 cells receiving one of each kind of chromosome necessary for perfect 

 development are small; and as a result the individual develops abnor- 

 mally. Bridges (5) has shown that in the fruit fly Drosophila ampelo- 

 phila certain variations from the normal chromosome combinations 

 have a definite effect upon the development of the zygote. Zygotes 

 containing 3X chromosomes die, while those containing 2X and a Y 

 chromosome develop normally. Male individuals may develop which 

 contain an X but no Y chromosome, but were found to be entirely sterile; 

 while those zygotes which received only a Y chromosome died as did 

 also those which received 2Y chromosomes, but no X. Zygotes con- 

 taining 2Y chromosomes plus an X, however, were able to develop into 

 normal males. Apparently the presence of an X chromosome is neces- 

 sary for the development of an individual, while the presence of a Y in 

 males is necessary if the male is to be fertile. 



In plants the evidence for the dependence of development upon 

 chromosome combinations is becoming indisputable if the Mendelian 

 interpretation of the inheritance of factors is admitted. In F^ progeny 

 of hybrids, if the parents are homozygous, there is generally as much 

 uniformity as is shown by either parent, while the Fj progeny shows a 

 wide range of types, often overstepping the limits of the parents. If 

 such a variety of types with regard to hardiness, rust resistance, adapta- 

 bility to various regional and soil conditions, and vigor of the individual 

 plant are produced in the 2X generation as the result of new chromo- 

 some combinations, why is it not possible for a similar series to exist in 

 the iX generation with regard to the ability of the individuals to develop 

 in a given environment? 



