230 INTRODUCTION TO CYTOLOGY 



Another important feature of the reduction process should be noted 

 before proceeding further. In many cases, chiefly among animals, the 

 chromosomes appearing on the spindle of the first maturation mitosis are 

 not merely double, but quadruple. This is due to the fact that each of the 

 conjugating chromosomes is already longitudinally split, giving the 

 bivalent chromosome the form of a chromosome tetrad (not to be confused 

 with tetrads of cells or nuclei). The four constituents of the chromosome 

 tetrad are known as chromatids, and are distributed by the two matura- 

 tion mitoses to the four resulting cells. It is thus seen that in the case of 

 chromosome tetrads the lines of separation for both maturation mitoses 

 are marked out in the prophase of the first. It should be borne in mind 

 that one of these lines represents a plane of chromosome conjugation, 

 and the other a plane of true longitudinal splitting. When the chroma- 

 tids separate along the conjugation plane reduction occurs, whether this 

 be at the first or second mitosis, and when separation along the plane of 

 splitting occurs the mitosis is equational, as in somatic division. 



MODES OF CHROMOSOME REDUCTION 

 In all cytology there is scarcely a subject upon which there has been 

 entertained so great a variety of opinion as upon the question of the 

 exact behavior of the chromosomes during the meiotic phases. Entirely 

 aside from the theoretical interpretations placed upon the process of 

 maturation, cytologists have yet failed to arrive at any universally 

 accepted conclusion regarding all the structural changes which occur. 

 This diversity of opinion is due in part to the complexity of the process 

 and the difficulty of interpreting its various stages, some of which fail 

 to stand out clearly in preparations made by our available methods. 

 On the other hand, a great variety of organisms have been studied, and 

 these undoubtedly differ considerably in the details of the reduction 

 process, so that agreement in all particulars is not to be expected. The 

 attempt has too often been made to apply universally an interpretation 

 founded upon a study of one or two organisms. Certain essential fea- 

 tures of meiosis may be expected to show close agreement in all organisms 

 reproducing sexually, as Strasburger pointed out, but it is evident that 

 there is no full correspondence as regards the exact manner in which the 

 essential changes are accomplished. In the following pages are given 

 brief descriptions of a few representative interpretations advanced by 

 various cytologists. 1 



The two interpretations of reduction which have been most conspicu- 

 ous in the literature of recent years are diagrammed in Figs. 83 and 89. 



1 No attempt can be made in a work of this scope to give a complete summary 

 and classification of all the interpretations that have been put upon the maturation 

 phenomena. Only enough will be presented to afford a starting point for a study of 

 this complex subject. For a review and criticism of all views expressed up to 1910 

 see Gregoire's two invaluable works (1905, 1910). A useful list of works on somatic 

 and heterotypic mitosis in angiosperms is given by Picard (1913). 



