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bulletin: museum of comparative zoology. 



III. Discussion ....... 



A. Synapsis and the maturation divisions . 



a. Results from Orthoptera 



b. Recent work on synapsis 



B. Individuality ..... 

 a Constancy of metaphase chromosomes 



1. Constancy in number 



2. Constancy in size and shape . 

 b. Persistent organization of chromosomes 



1. The selected chromosomes 



2. The heterochromosomes . 



3. Plasmosomes and nucleoli . 



4. Persistence of chromosomes between mitoses 



C. Chromosomes and heredity . 



a. Mendelism and maturation 



b. Some experimental evidence 



D. Summary of conclusions 



IV. Bibliography .... 

 V Explanation of plates 



Page. 



95 



95 



95 



98 



105 



105 



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106 



110 



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118 



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I. INTRODUCTION. 



A. Outline of the Problems. 



Two of the most important subjects which have claimed the atten- 

 tion of cytologists for many years are the two named in the subtitle 

 of this paper. Every species of animals and plants is thought to have 

 a definite number of chromosomes, which is characteristic for the 

 species. In the process of maturation this typical, or diploid, number 

 becomes reduced so that each functional gamete contains only half 

 that number, the haploid number. It is generally believed that the 

 process of reduction is initiated by a pairing of the chromosomes in 

 the prophase of the first maturation division. It is also generally 

 admitted that of the two chromosomes which united to form a single 

 pair, one has been derived from the maternal, and the other from the 

 paternal ancestor, and that these become separated again at one of the 

 two maturation divisions. But there has been a considerable amount 

 of disagreement as to how the pairing of the chromosomes takes place, 

 and also differences of opinion as to which of the two maturation divi- 

 sions results in their separation. 



As to the process by which the pairing of chromosomes is accom- 



