Heredity and sex . ^ m= ^ ^ I^- ;o. Fig. 49. — lUustrating chiasma-type theor\-. 1 and 2. from Tritoncristatus. 3—46. chromosomes of Batracoseps att^nuatu^. Xote especiallythe chiasma shown in 13. (After Janssens). 94 HEREDITY AND SEX (when two are present as in the female), hke all otherchromosomes, unite in pairs at the synaptic recognized method of uniting is for like chromosomesto come to lie side by side. Before they separate, as they do at one of the twomaturation divisions, each chromosome may be seento be spht throughout the length. Thus there are. tig.}- Fig.
Heredity and sex . ^ m= ^ ^ I^- ;o. Fig. 49. — lUustrating chiasma-type theor\-. 1 and 2. from Tritoncristatus. 3—46. chromosomes of Batracoseps att^nuatu^. Xote especiallythe chiasma shown in 13. (After Janssens). 94 HEREDITY AND SEX (when two are present as in the female), hke all otherchromosomes, unite in pairs at the synaptic recognized method of uniting is for like chromosomesto come to lie side by side. Before they separate, as they do at one of the twomaturation divisions, each chromosome may be seento be spht throughout the length. Thus there are. tig.}- Fig. 50. — Chromatin filaments in the amphitene stage from spermato-cytes of Batracoseps. (After Janssens.) formed four parallel strands each equivalent to achromosome — the tetrad group. At this time Jans-sens has found that cross unions between the strandsare sometimes present (Fig. 49). In consequencea strand is made up of a part of one chromosome anda part of another. Whether this cross union can bereferred to an earlier stage — at the time when the twolike chromosomes come together, when they can be THE MENDELIAN PRINCIPLES OF HEREDITY 95 seen to twist around each other (Fig. 50) — is notcertain; but the fact of the existence of cross connec-tions is the important point. A consequence of thiscondition is that the chromosomes that come out ofthe tetrad may represent different combinations ofthose that united to form the group. On the basisof this observation we can explain the results of associ-ated inheritance. For, to the same extent to whichthe chromosomes
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Keywords: ., boo, bookcentury1900, bookdecade1910, booksubjectsex, bookyear1913
