228 N. M. STEVENS. 



parasynapsis and split across the twists in preparation for mitosis, 

 giving daughter chromsomes which contain both maternal and 

 paternal chromatin. 



In Ceuthophilus the parasynapsis stage of Fig. 27 is intimate 

 enough and long enough to favor the supposition that it is a 

 true conjugation involving exchange of material particles or of 

 chemical substances (genes), but there is no evidence of any 

 splitting of Morgan's chiasma type. All of the evidence indicates 

 that homologous paternal and maternal chromosomes twist 

 together in parasynapsis and untwist in the prophase of the 

 first maturation mitosis. In the flies and mosquitoes (Stevens, 

 '08, '10, f n) we have examples of even more pronounced para- 

 synapsis than in Ceuthophilus, but so far as I have seen, the 

 indications are that the chromosome pairs twist up in synapsis 

 and untwist in prophase much as in Ceuthophilus; i. e., an op- 

 portunity for interchange of genes between homologous maternal 

 and paternal chromosomes is furnished by the observed phe- 

 nomena of parasynapsis in these forms, but no evidence of such a 

 chiasma type of splitting after synapsis as is suggested by Morgan 

 ('IT) to account for the results of his breeding experiments with 

 Drosophila. Such an exchange of parts of chromosomes as that 

 described by Janssens ('09) might of course occur without being 

 detected, at almost any point in the process of twisting or un- 

 twisting of the pairs, since the time element is not determinate 

 in fixed preparations. 



Moreover, it seems to me that, in view of the great range of 

 variation in the phenomena of conjugation and segregation of the 

 chromosome in the maturation of germ cells, cytological evidence 

 from one form cannot safely be taken to serve as the basis of a 

 theory or hypothesis to account for the experimental results on 

 another form, but cytological and experimental work on the same 

 form must go hand in hand, in order that any safe conclusions 

 may be drawn from the results. 



There seems to be no question but that synapsis, or conjugation 

 of the chromosomes is the most difficult phenomenon connected 

 with the maturation of the germ cells, to interpret correctly, and 

 doubtless earlier parasynaptic stages have been overlooked 'in 

 some cases where telosynapsis alone has been described in con- 



