258 INTRODUCTION TO CYTOLOGY 



physical basis of heredity. Special attention will be devoted to this 

 question in Chapter XVII. 



Chromomeres. An important r61e has been attributed to the chro- 

 momeres by many students of synapsis. Allen (1905), for example, 

 maintained that the fusion of the leptotene threads in Lilium involves a 

 fusion of their chromomeres, the subsequent division of the fused chro- 

 momeres initiating the resplitting of the pachytene thread. Allen found 

 the chromomeres to be composed of still smaller chromatic elements, 

 and offered various suggestions concerning the manner in which the re- 

 splitting of the pachytene thread might be supposed to effect a redistri- 

 bution of the "idioplasms." That chromosome conjugation is primarily 

 a conjugation of small chromatic elements within the chromosome was 

 held by Strasburger (1904, 1905) and the Schreiners (1906). The visible 

 chromatin granules, or "pangenosomes," were conceived by Strasburger 

 to represent complexes of "pangens" such as were postulated by de Vries, 

 conjugation involving an interchange of these latter units. 



The chromomere interpretation has been adversely criticised by 

 Gr6goire (1907, 1910) on the basis of further evidence obtained from a 

 study of the chromatic structures themselves. This author points out 

 several serious objections to the view that the chromomeres are auton- 

 omous bodies, and concludes that they are rather to be regarded simply 

 as swellings or thicker portions of the chromatin thread, such thick por- 

 tions remaining as the thread undergoes a stretching which is not uni- 

 formly resisted at all points. Their frequently striking correspondence 

 or paired arrangement in the synaptic threads is explained as the result 

 of the response of the two closely associated threads to the same stretch- 

 ing force. This interpretation is also shown to account for the variability 

 in the dimensions of the chromomeres, their tapering form, the often 

 reported absence of correspondence between the chromomeres of the 

 two threads, and various other aspects. Wenrich (1916, 1917), on the 

 other hand, has found that in Phrynotettix (Fig. 155) the chromomeres 

 show a remarkable individual constancy in size and position in a given 

 member of the chromosome complement, not only in the various cells of 

 a given individual, but also in those of different individuals. These 

 facts strongly suggest an autonomy of the bodies in question. 



Because of their great theoretical importance (see Chapter XVII) it 

 is to be regretted that after such a large amount of research so many 

 points regarding the process of synapsis should remain in such an un- 

 settled state. It is hoped that further refinements in microtechnique 

 may remove some of the obscurity which at present surrounds them. 



OTHER OPINIONS ON THE HETEROTYPIC PROPHASE 



Although the phenomena of the heterotypic prophase, particularly 

 synizesis and synapsis, are generally looked upon as normal occurrences 



