146 



INTRODUCTION TO CYTOLOGY 



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virtually a group of separate elementary nuclei, or karyomeres (Fig. 75). 

 Each chromosome may even have its own elementary spindle at the time 

 of mitosis; this is known as merokinesis (Reuter, 1909). It has also been 

 observed that when the chromosomes lie in a certain relative position 

 when the telophasic reticulum is formed, they tend to appear in the same 

 position in the following prophase. ^^ It is therefore inferred that, even 

 when the whole reticulum appears like a single unit under the microscope, 



the integrity of the constituent chromonemata is 

 nevertheless somehow -preserved. This idea is 

 supported by those cases in which certain por- 

 tions of certain chromosomes remain dense and 

 deeply chromatic ("heteropyknotic") and hence 

 distinguishable while the remaining portions 

 develop an otherwise uniform reticulum (Fig. 80). 

 Furthermore, when two species with chromosomes 

 of unlike size are crossed, the chromosomes of the 

 two parents can be distinguished in the dividing 

 nuclei of the hybrid (Fig. 206). Often the 

 parental groups do not intermingle but tend to 

 remain rafher distinct through several embryonal 

 cell generations-"^ (Fig. 143). 



Many years ago Boveri was led by the results 

 of his brilliant researches on echinoderm eggs to 

 conclude that the number of chromosomes arising 

 from the reticulum in the prophase is directly and 

 exclusively dependent upon the number which 

 built it in the preceding telophase; this, he con- 

 tended, finds its most logical explanation in a 

 genetic chromosomal continuity. These conclu- 

 sions have been confirmed repeatedly by researches 

 phase. B, late telophase, on the chromosomes of hybrids, by observations 



The heteropyknotic re- ,, i • , i , ,1 ■ n e 



gions are numbered, ^u ceils Subjected to the mtiuencc 01 various 

 Compare Fig. 220. {After agcucies causing aberrant chromosome behavior, 



and by critical analyses of the chromosome com- 

 plements in organisms showing unexpected departures from the normal 

 chromosome number and morphology. 



The chromosome is not a body with unchanging form and composition. 

 In the course of the nuclear cycle it passes through a complicated series of 

 chemical and structural alterations, as does the organism itself during 

 its individual life cycle. At certain stages, notably the metabolic stage 



21 E.g., in the segmenting egg of Ascaris, according to Boveri (1887a, 1891, 1909c) 

 and Herla (1893). See also p. 125. 



" Haecker (1895c), Riickert (1895), Conklin (1897, 1901a), Moenkhaus (1904), 

 Tennent (1912), Morris (1914), Richards (1916), B. G. Smith (1919, 1929) 



B 



'■<-^ 



S 



Fig. 80. — Heteropyk- 

 nosis in chromosomes of 

 Pellia epiphylla. A, pro- 



ff 



