. The physical basis of heredity . 41, d). They are herethe two non-crossover strands, but if the two strandsthrown to the left had been thrown to the right the twocrossover strands would cross over. The scheme is essen-tially the same as the chiasma of Janssens, but the strandsthat cross may or may not (as here) represent the cross-over strands. CROSSING OVEE AND CHROMOSOMES 103 The next two figures (Fig. 42, a,h) show how Robert-son and Wenrich interpret the crossed threads, that theyhave observed in the spermatogenesis of some of thegrasshoppers. The four strands are represented as con-juga
. The physical basis of heredity . 41, d). They are herethe two non-crossover strands, but if the two strandsthrown to the left had been thrown to the right the twocrossover strands would cross over. The scheme is essen-tially the same as the chiasma of Janssens, but the strandsthat cross may or may not (as here) represent the cross-over strands. CROSSING OVEE AND CHROMOSOMES 103 The next two figures (Fig. 42, a,h) show how Robert-son and Wenrich interpret the crossed threads, that theyhave observed in the spermatogenesis of some of thegrasshoppers. The four strands are represented as con-jugating side by side in Fig. 42, a. When the strandsbegin to open out preparatory to the first spermatocytedivision the two maternal separate from the paternal at theends of the tetrad, while in the middle of the tetrad theopening up involves the separation of a maternal and apaternal strand from a maternal and a paternal. Inother words, the tetrad opens up in two planes at rightangles to each other. This scheme also gives an appa-. Fia. 43.—Scheme showing crossing over involving both strands of each chromosome. rent crossing of the strands at the level where the open-ing out in one plane passes over into the opening outin the other plane, but there has been no real crossingover of the strands in the sense of interchange betweenthem. Theoretically this explanation is sound, andmoreover seems to be supported by observations incases where the maternal and the paternal strands canbe identified. The results undoubtedly show that theoccurrence of crossed threads in cases where the splitoccurs in two planes does not necessarily imply thatcrossing over has taken place; but, on the other hand, ashas been shown (in Fig. 41) a similar figure may alsonecessarily result after crossing over of the threads. Ina word, the crossed-strand stage is not ipso facto evidencethat it must have come about according to Robertsons 104 PHYSICAL BASIS OF HEREDITY scheme. It should also be observed that the
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectheredit, bookyear1919
