394 INTRODUCTION TO CYTOLOGY 



without recourse to the hypothesis of chiasmatypy, and that the observa- 

 tions and figures of Janssens do not prove the existence of that phe- 

 nomenon. Robertson, for example, holds that the "chiasma" figures are 

 more simply explained as the result of a tendency on the part of the four 

 chromatids to open out partly along the conjugation plane and partly 

 along the plane of splitting, without any actual breaking and recombina- 

 tion (Fig. 156). Wilson (Wilson and Morgan 1920), however, thinks it 

 "highly probable that the cytological mechanism of crossing-over must be 

 sought in some process of torsion and recombination in the earlier stages 

 of meiosis perhaps during the synaptic phase or slightly later and that 

 this process may leave no visible trace in the resulting spireme-threads." 



During the time when the homologous chromosomes in the form of 

 slender threads are twisted about each other in the early prophase of the 

 heterotypic mitosis there is abundant opportunity for the required break- 

 ing and union to occur, and appearances often lend themselves well to 

 such an interpretation. If the side-by-side position is not assumed until 

 later in the prophase (Scheme B, Chapter XI) the time during which such 

 interchanges might occur is much shorter. But it is a matter of extreme 

 practical difficulty in either case to determine whether or not the plane of 

 separation is the same as the plane of union at a given crossing point. 

 The forces controlling the breaks and recombinations as well as the fre- 

 quencies with which they occur in the different chromosome pairs are 

 even more difficult to imagine. The difficulty of accounting for these 

 phenomena, however, does not weigh heavily as an argument against 

 their occurrence. Whether or not chiasmatypy actually takes place is a 

 question which must be settled primarily by direct evidence, and the need 

 for careful search for such evidence cannot be too strongly emphasized. In 

 the opinion of the cytologist the behavior of each chromosome as a whole 

 must be much more thoroughly known before cytological interpretations 

 of the phenomena of inheritance involving any smaller units which the 

 chromosome may contain can be regarded as more than hypotheses, 

 valuable as these hypotheses may be in the correlation of genetic data. 



At this point it may be well to recall (see Chapter XI) that all cytolo- 

 gists do not agree that the synaptic mates maintain such an independ- 

 ence (except at crossover points) as is presupposed by the advocates of 

 the chiasmatype theory. A number of observers have reported an actual 

 fusion of the conjugating threads, the resulting pachytne thread sub- 

 sequently splitting, probably along the line of this fusion. It may 

 accordingly be suggested, in line with the hypotheses discussed by Allen 

 (1905), that if the threads actually carry or consist of discrete units or 

 genes, and if these units do not themselves fuse during the process, such 

 a resplitting of the pachytene thread, if not wholly along the fusion plane 

 because of a twist of the thread or of the plane itself, would bring 

 about a redistribution of the genes as effectively as would the chiasmatype 



