386 INTRODUCTION TO CYTOLOGY 



Between these two extremes lie other views, the most suggestive of them 

 being that proposed by Janssens (1909). 1 



The theory of Janssens in its simplest possible form may be stated as 

 follows. The members of the conjugating pair twist about each other 

 and come into very intimate association at certain points. When they 

 again separate a break occurs at the point or points of closest contact, 

 but along a new plane, so that each of the two separating chromosomes 

 is made up of portions of both conjugating members (Fig. 149). This 

 process is known as chiasmatypy or crossing over. Such a behavior might 

 occur at various stages in the heterotypic prophase: most probably it 

 takes place at an early stage, when the conjugating chromosomes are in 

 the form of simple thin threads (Fig. 83). In other cases it may take 

 place at a later stage, when, in the case of animals, each of the chromo- 

 somes has split preparatory to the second mitosis, forming a tetrad of 

 chromatids. Here the crossing over may occur between only two of the 

 four chromatids (Fig. 150, Janssens's typical case; see also Fig. 151), 

 or between all four. If only two of the four chromatids are concerned, 

 only two of the resulting gametes (or spores) will be "crossover gametes" 

 (or spores), as in Fig. 150; whereas, if the crossing over takes place between 

 all four of the chromatids, or between the two yet unsplit threads in the 

 earlier prophase, all four of the gametes (or spores) will be "crossover 

 gametes" (or spores). 



Application of the Chiasmatype Theory to the Problems of Linkage. 

 It is the above interpretation of the nature of. chromosome conjugation 

 that lies at the basis of the work of Morgan and his students on Drosophila. 

 As already pointed out, these workers have found good evidence for the 

 conclusion that each chromosome is responsible for a certain group of 

 characters, the members of the group showing a strong tendency to remain 

 associated because their genes are borne by a common carrier. They 

 further believe that the evolution of new character groupings has been 

 brought about not only through the crossing of different hereditary 

 strains, but also through the evolution of chromosomes with new con- 

 stitutions by the process of crossing over. On the basis of the frequencies 

 in which the new types of grouping occur the relative positions (loci) 

 of the genes for the different characters have been plotted in the 

 chromosomes. 



The above points are illustrated in Fig. 152, which summarizes what 

 is supposed to have occurred in a certain series of crosses between flies 

 with yellow body, white eyes, and miniature wings, and flies with gray 

 body, white eyes, and long wings. In the cells of the hybrid there is 



1 Janssens has recently (1919a6) published an outline of his views of the maturation 

 phenomena in Orthoptera in which he again makes use of the chiasmatype interpreta- 

 tion. His results are discussed in some detail from the cytological and genetic points 

 of view by Wilson and Morgan (1920). 



