116 INTRODUCTION TO CYTOLOGY 



It was observed by Agar (1912) in a fish that it is at an achromatic 

 "constriction" that the chromosome estabHshes connection with the 

 spindle in the metaphase and separates first in the anaphase. This 

 is now known to be true of chromosomes generally. Hence the region 

 functioning in this manner has been variously called the "fiber-attach- 

 ment point," "insertion region," "primary constriction," "kinetic 

 constriction," "attachment constriction," and "Trennungstelle" (separa- 

 tion place). ^ Chromosomes with apparently terminal spindle attach- 

 ment are said to be telomitic; those with attachment elsewhere are 

 atelomitic (Carothers, 1917). 



As the result of the definite location of the spindle-attachment 

 region, each atelomitic chromosome has two main segments, or "arms," 

 with a characteristic length ratio. This is easily seen in side views of 

 anaphase figures, also in polar views of metaphase ones if the chromosome 

 is not too long. Such a chromosome passes poleward as a V or V, the 

 apex leading, whereas a telomitic one appears as an unbent rod. The 

 same mitotic figure may include all types. It has been held by certain 

 investigators^ that all chromosomes are probably two-armed, "telomitic" 

 ones having an extremely minute second arm which easily escapes 

 observation. Supposedly telomitic chromosomes have been shown in 

 some instances to have their attachment region slightly back from the 

 end, and in other cases a minute body seen at the end may represent the 

 second arm; but that strictly one-armed chromosomes do not exist is 

 at present improbable. What may have been the primitive location 

 of the attachment, and how other types have arisen, are interesting 

 subjects for speculation. 



The structure of the spindle-attachment region has been the subject 

 of some study. It may appear clearly in the prophase as a region in 

 which highly stainable matter is lacking. It may be straight sided or 

 somewhat swollen in some preparations (Fig. 152), and in certain instances 

 it is known to have a characteristic length (Fig. 188). It appears to 

 be homogeneous except for minute granules^ at the middle point (Figs. 

 62, 63). The reaction which "attaches" the chromosome to the spindle 

 seems to center in these "kinetic bodies," for it is from them that the 

 strongly developed "spindle fibers" appear to extend in fixed prepara- 

 tions, and it is they which take the lead in the anaphasic movement. 

 They have been observed at the extreme end of the attachment region 



^ The convenient term kinetochore ( = movement place) has been suggested to the 

 author by J. A. Moore. The use of this term is recommended. 



^ S. Nawaschin (1916), Delaunay (1929), Heitz (1928a), Lewitsky (1930, 1931a). 

 Chromosomes with apparently equal arms are termed isobrachial and unequal armed 

 ones heierobrachial by Sorokin (1929). Lewitsky (1931a) calls those with a small, 

 rounded second arm "cephalobrachial," or "headed" chromosomes. 



' Observed by S. Nawaschin (1913, 1916), Belling (19286), Belaf (1929a), Tran- 

 kowsky (1930a), Sharp (Fig. 63), and others. 



