THE REDUCTION OF THE CHROMOSOMES 



231 



Nearly all of the accounts of reduction now appearing, especially those 

 given by botanists, conform in general to one or the other of these two 

 schemes, though they vary greatly in detail. Both theories have been 

 upheld by competent observers, and it may be possible that both modes 

 of reduction actually occur; but the same objects have been so differently 

 described by the two opposing schools that it seems very probable that 

 interpretation is chiefly responsible for the persistent diversity of opinion. 

 For convenience the two theories will be referred to as Scheme A and 

 Scheme B. 



UL7TONEMA 



STfltPSINLMA 



FIG. 83. The method of chromosome reduction according to Scheme A. 

 Explanation in text. 



Scheme A. The first of the two main interpretations of reduction 

 came into prominence in 1900 and shortly after, when von Winiwarter 

 (1900), Gregoire (1904, 1907, 1909), A. and K. E. Schreiner (1904-1908), 

 and Berghs (1904, 1905) applied it to the phenomena observed by them 

 in several animals and plants. Its essential points are as follows (Figs. 

 83-88) : 



At the beginning of the heterotypic prophase the nuclear reticulum, 

 without breaking down into such distinct elementary nets or alveolar 

 units as are seen in the somatic prophase, takes the form of long slender 

 threads (leptotene or leptonema stage). 1 During the very early prophase 



1 The terms leplotene, synaptene, pachylene, and diplotene were proposed by von 

 Winiwarter (1900); 'leptonema, zygotene, pachynema, and strepsinema by Gre'goire 

 (1907); amphitene by Janssens (1905); strepsitene by Dixon (1900); diakinesis by 

 Haecker (1897); synapsis by Moore (1896); synizesis by McClung (1905); and meiosis 

 by Farmer and Moore (1905). The terms ending in -tene are ordinarily used as 

 adjectives. 



