130 Morphologie etc. — Varietäten etc. 



Vorgang; sie erscheinen als schwarze, feingekörnelte Linien, die 

 in bogigera Verlaufe die dicken Zellwände durchsetzen. Bei Fran- 

 gula Alnus aber — wegen der Zartheit der Plasmodesmen — muss 

 eine Nachbehandlung eintreten, die in folgendem besteht: die auf- 

 geklebten Mikrotomschnitte bleiben 8 — 10 Tage vor Lösung des 

 Paraffins in diffusem Tageslichte liegen; die Schwärzung von Plasma 

 und der Plasmodesmen wird dadurch verstärkt. Eine Einbettung 

 in Kanadabalsam behufs Fixierung des Präparates bewährte sich 

 bisher nicht, doch wird eine solche vielleicht doch möglich sein. 



Matouschek (Wien). 



Muller, H. J., The mechanism of crossing-o ver. (American 

 Naturalist. L. p. 193—221, 284—305, 350-366, 421—434. 1917.) 



The writers summary of results obtained runs as follows: 



1. Recent results complete the parallelism between factorgroups 

 and chromosomes in Drosophila. This strengthens the evidence that 

 Separation of linked factors is due to an interchange between chro- 

 mosomes. 



2. The Chief gaps in the information regarding the total fre- 

 quency of interchange in the different groups have been filled and 

 it is found that the usual total frequencies of Separation correspond 

 to the lengths of the chromosomes. This constitutes specific evi- 

 dence that crossing-over is the method of interchange between the 

 chromosomes, and that the frequency of crossing over between 

 factors is determined by their distance apart in the chromosome. 

 It Supplements the other evidence for these conclusions that had 

 previously been found by Sturtevant in the linear manner or 

 linkage of the factors. 



3. It seems uncertain whether crossing-over ocours in the strep- 

 sinema stage, as concluded by Janssens, or earlier in synapsis. 

 The cytological evidence at present at band would seem insufficient 

 to settle this point. Possible tests for various alternative mechanics 

 of crossing-over are proposed. 



4. In Order to study the nature of crossing-over by means of 

 "interference", Stocks were made up that differed in regard to many 

 factors. Females heterozygous for 22 pairs of factors were thus ob- 

 tained, and a special method was devised for testing their Output. 

 Other special methods for obtaining multiple Stocks, and for elimi- 

 nating discrepancies due to differential viability, have also been 

 presented. 



5. The results have been arranged in the form of a curve 

 showing the amount of interference for various distances. The results 

 thus far obtained, confirm those obtained by less exact methods, 

 and also give evidence that interference decreases gradually with 

 distance from a point of crossing-over; this, taken together with 

 certain evidence from non-disjunction, lends some probability to the 

 view that crossing-over occurs at an early stage in sjmapsis. 



6. A Gase of crossing-over in an embryonic cell of a male is 

 reported. 



7. Incidentally, the experiments have afforded an extensive 

 lest of Castle's assuraption of contamination of factors by their 

 allelomorphs. Outcrossing in each generation for 75 generations has 

 failed to change any of the factors. M. J. Sirks (Bunnik). 



