32 MITOSIS:. THE CONSTANCY OF THE CHROMOSOMES 



tissues for all the chromosomes of a complement except for such as 

 may be less in bulk than a sphere of the specific diameter {v. Fig. 7). 



Variation in the apparent diameter of the chromosome is due to, 

 first, the pairs of chromatids being seen sometimes endways-on, i.e., 

 in the same line of vision, as is always the case near the centromere in 

 polar views, and sometimes sideways-on, when the chromosome will 

 be just twice as broad ; and, secondly, the occurrence of constrictions 

 {v. infra). 



Now each of the cylindrical rods is the product of a linear 

 contraction of the threads observed in early prophase. The 

 extent of this contraction can be approximately estimated 

 from measurements. The rather fragmentary estimates given are 

 largely derived from observations of successive stages at meiosis, 

 where the contraction at metaphase is greater than at mitosis 

 (cf. Bellevalia), but they indicate the extent of the changes undergone 

 during the prophases of both types of division (Table 3) . 



The degree of contraction at metaphase is, however, subject to 

 genetical control, as will be seen later (7;. Ch. Ill), and may therefore 

 vary considerably in different races or species. 



In this process of contraction we may probably distinguish two 

 kinds of change in succession. First, there is the change which 

 renders the chromosomes visible at the end of the resting stage. 

 This may be regarded as a dehydration of previously more dispersed 

 chromatin material, associated (according to Sakamura, 1927) with 

 an increase in hydrogen-ion concentration in the nuclear sap. 



Secondly, there is a linear contraction of the thread such as might 

 be regarded merely as due to successive particles, the chromomeres 

 observed at meiosis coming to lie closer together. But since the 

 separation into chromomeres is probably to some extent an artefact 

 (although characteristic and significant) such a simple contraction 

 need not necessarily be distinguished from a spatial rearrangement 

 of the thread. The rearrangement might be supposed a priori to 

 be orderly or haphazard. Since each chromatid is uniformly 

 cylindrical at metaphase, there seems no arrangement that can be 

 assumed but an orderly one, and none of this kind but a spiral. 

 Such an arrangement was first observed by Baranetsky in 1880, 

 and his observations have since been generally confirmed whenever 



