166 FUNDAMENTALS OF CYTOLOGY 



li()iii()l()jj;(nis chromosomal elements occurs in the first two divisions but 

 not in the third. This is of special interest in connection with a conten- 

 tion that in some species there are two nuclear fusions in the cycle, one 

 in the ascogonium and another in the ascus, and that a double reduction 

 compensates for this in the three ascus divisions. It now seems evident 

 that if such a process occurs it is very rare. 



An interesting feature of nuclear division in asci is the intranuclear 

 character of the achromatic figure. As the nucleus enlarges in the pro- 

 phase, the spindle develops in the karyolymph with its poles at the nuclear 

 membrane. Asters, when present, lie in the cytoplasm. The membrane 

 may remain intact until anaphase or even throughout the entire nuclear 

 cycle, constricting between the daughter chromosome groups at the close 

 of division. Such figures are also present in certain other plant and 

 animal cells. 



Conclusions. — The series of three chapters now being concluded 

 should serve several purposes. 



1. It should furnish cytological pictures, albeit sketchy ones, of the 

 reproduction and life cycles of organisms representing many natural 

 groups. These constitute an essential part of the working cytologist's 

 background. 



2. It should indicate the great variet}^ of materials available for 

 research projects of various kinds. The success of a project may depend 

 largely or entirely upon a wise selection of an organism or tissue as a 

 basis of investigation, and there are many of these to choose from in 

 nature. From what has been set forth in these chapters it should be 

 obvious that one can find materials peculiarly suited to the study of 

 such problems as the role of asters in cytokinesis, the causes and effects 

 of parthenogenesis, the role of cells as units in tissues, the relative effects 

 of monoploidy and diploidy, the process of secretion in cells, the role of 

 the nucleus in development and heredity, and so on. 



A single striking illustration of this is afforded by certain fungi 

 described in the preceding section. Cytologists and geneticists have 

 long wished to know if or in what manner the effect upon development 

 exerted by a fusion nucleus after syngamy differs from that of the nuclei 

 before the fusion; in other words, does the association of two genomes 

 within a common nuclear membrane result in activity differing from 

 that of two unfused monoploid nuclei lying in the common cytoplasm? 

 Material nicely suited to the solution of this problem has been found in 

 the peculiar dikaryophase of basidiomycetes and ascomycetes. In 

 Penio-phora and Neurospora, representing these two groups respectively, 

 evidence is accumulating that will yield an answer to this important 

 question. At the present time it appears that at least some of the 

 activities of a fusion nucleus are duplicated by the dikaryon, or unfused 



