PREFACE. 



The organization of the resting nucleus and its relation to the proc- 

 esses of nuclear fusion and division are the main problems with which 

 I have been concerned in continuing my studies on the mildews. The 

 doctrine is commonly accepted that the chromosomes are in a special 

 sense the physical basis of heredity, but their relation, especially in the 

 resting nucleus, to the mechanism of cell division and to the centrosome 

 is still undetermined. The evidence for the so-called " individuality " 

 of the chromosomes has been developed almost entirely from a study of 

 their appearance in mitosis. In Phyllactinia I have been able to show 

 that the material of each chromosome is in permanent connection with 

 the central body throughout the stages of nuclear fusion and the resting 

 condition, as well as in mitosis, thus affording an explanation of the 

 means by which the permanence of the chromosomes is secured and 

 throwing further light on the mechanics of nuclear division. The 

 nucleus is thus shown to have a permanent polar organization ; and the 

 central body is a permanent structure in the cell which determines the 

 point of special connection between the nuclear contents and the cyto- 

 plasm in the mildews. The regularly recurring triple nuclear division 

 in the ascus has been the most serious objection to the acceptance of the 

 frequently proposed view that the ascocarp is a sporophyte and the 

 ascus a spore mother cell. But with the facts of the nuclear history in 

 Phyllactinia here brought out, showing that each chromosome is a per- 

 manent unit through the processes of nuclear fusion as well as division, 

 and that synapsis and a numerical reduction of the chromosome number 

 occur in the ascus, it becomes plain that the triple division in the ascus is 

 a natural correlative of the occurrence of two nuclear fusions in the de- 

 velopment of the ascocarp. The evidence thus becomes very strong that 

 we have a true alternation of generation in the Ascomycetes. In the 

 light of the principle of the " nucleo-cytoplasmic relation " it is also 

 evident that the size, method of development, and functions of the ascus 

 all indicate that the nuclear fusion which occurs in it is a correlative of 

 the general vegetative and growth processes involved in maintaining 

 the nucleo-cytoplasmic equilibrium in a cell of such relatively gigantic 

 size as the ascus. 



The presence of typically differentiated and functional sexual cells in 

 Phyllactinia, together with the abundant evidence which has recently 



