44 AN INTRODUCTION TO MODERN GENETICS 



nucleus appears very similar to an interphase nucleus. Darlington^ has 

 shown that a similar uncoiling of the chromonema occurs in telophase, 

 and further provides a very elegant demonstration of the continuity of 

 the chromosomes, similar in principle to Boveri's above, by showing 

 that the spirals of one telophase can be seen in the following prophase. 

 It is possible that the thread-like chromonema may be so flexible as 

 to make little difference to the apparent viscosity of the nuclear con- 

 tents and be insufficiently rigid to be demonstrated by the ordinary 

 methods of fixation. But the structure of the interphase chromosomes 

 is of more or less molecular dimensions (p. 377) and the model of a 

 coherent thread, appropriate at larger dimensions, may be very mis- 

 leading here. Moreover no description of the structure can be complete 

 which ignores the fact that during the interphase the chromosomes 

 become duplicated or split, and this is a process about which we know 

 very little more than that it occurs. 



6c. The Mechanism of Segregation 



The chromosomal mechanism which is invoked to explain the 

 segregation of genes is the pairing of like chromosomes in zygotene and 

 their separation one to each daughter cell; the chromosomes are 

 assumed to be in so far dissimilar that each contains a different allelo- 

 morph. The evidence as regards separation to the two poles is obvious, 

 since it follows directly from observation, though it is not immediately 

 obvious whether the two allelomorphs are separated at the first division 

 or the second (p. 104). The crucial evidence we require to provide a 

 basis for the whole theory is evidence that the two chromosomes which 

 pair are similar or homologous ones. 



The most direct evidence is derived from the observation of the 

 similarity of pairing chromosomes in those organisms in which each 

 chromosome is individually recognizable (e.g. Crepis). However it is 

 usually difficult to make out the shape of chromosomes at the pairing 

 stage (zygotene) because they are still uncontracted and tangled with 

 one another. In some organisms the chromosomes show a row of small 

 swellings or chromomeres, and in prophase the pairing chromosomes 

 can be seen to correspond, not only in a general way but actually point 

 for point. A similar detailed correspondence is seen between the two 

 pairing members of the saHvary gland nuclei in Diptera (p. 100), and 

 although this pairing cannot be the samxe in every way as that found in 

 ^ Darlington 1935a. 



