DEPARTMENT OF EXPERIMENTAL EVOLUTION * 



C. B. Davenport, Director. 



The present year has marked an advance in that cytological, chemical, 

 and morphological studies on mutation and pigmentation have moved far 

 along converging lines. From several points of view new light has been 

 gained on the nature and control of sex. The studies on human heredity 

 have made substantial progress. 



DETAILED REPORTS ON SCIENTIFIC WORK. 



ORIGIN AND CYTOLOGICAL BASIS OF MUTATIONS. 



One of the most striking results of the year is a demonstration, more 

 complete than has been made hitherto, of the conclusion that has long been 

 held in a semi-speculative way, that the chromosomes of the cell contain 

 materials that determine the direction of development of the organism and 

 the details of its structure. The demonstration has been made in this wise : 

 Usually a given species has a definite and constant number of chromosomes 

 in all individuals that are capable of reproduction. But in Oenothera la- 

 marckiana and its mutants the number is variable. This fact was early 

 demonstrated in our cultures of the evening primrose by Miss Anne M. 

 Lutz. Thus she found 14 chromosomes in the tissue-cells of the form la- 

 mar ckiana, 15 chromosomes in the form lata, and 28 chromosomes in the 

 form gigas. Thus, each of these characteristic forms of the primrose, sep- 

 arated by marked differences of form and size, has its special condition of 

 chromosomes. The conclusion can hardly be escaped that each special con- 

 dition of the chromosomes determines the peculiarities of the form of the 

 adult body ; and, in general, within cultural limits, what the organism shall 

 develop into is determined by its germ-plasm. 



Miss Lutz's studies throw light specifically on the mutability of the even- 

 ing primrose. They indicate that the primrose is mutable just because in it 

 the mechanism for exact division of the determiners, at the ripening of the 

 germ-cells, is imperfect and irregular. The reason for this irregularity is, 

 in part, an inequality in the number of chromosomes that have come from 

 the two parents, so that there are one or more unpaired chromosomes whose 

 fate in the maturation of the sex-cells is undetermined and variable and, 

 consequently, results in variable combinations of determiners, and so in 

 variable progeny. The suggestion arises that "mutation" is always induced 

 by some irregularity in chromosomal division — a condition that may, prob- 



* Situated at Cold Spring Harbor, Long Island, New York. Grant No. 742. $37,477 

 for investigations and maintenance during 1912. (For previous reports see Year Books 

 Nos. 3-10.) 



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