Studies on the sex chromosomes of animals.

Meiosis in male spiders is distinguished by the multiple. sex chromosome mechanism whereby usually two or three X chromosomes move jointly to one pole at the first division in the absence of any y chromosome. The mechanism of segregation is, thus, fundamentally different from that usual in XY and XnY systems where co-orientation and segregation occur as a consequence of pairing, usually by chiasma formation between the X and Y chromosomes. The existence of this multiple system has been recorded by a number of authors but little attempt was made to explain the behaviour of the sex chromosomes until Revell (1947) reported a condition of \continuous polarisation\" in Tegenaria. life ascribed the regular X-segregation to a persistence of polar attraction after oachytene which leads to the bivalents and sex chromosomes being distributed in two groups about the centrosomes when these have separated. Subsequently on the formation of the spindle the bivalents successively move into the equatorial plane as their centromeres co-orientate the Ps occupying a position nearer to the pole to which they will move together at anaphase. Although a similar condition of continuous polarisation has been illustrated in Trochosa ruricola (Hackman 1948) and is evidently present in other species (ibid.; Wallace 1909) it does not occur universally in the spermatogenesis of spiders. For example Atau (1948) found that polarisation lapses at pachytene in Iranea reaumuri in the usual manner. The normality of the meiotic cycle in Pranea suggests that continuous polarisation in not the primary cause of joint x-segregation but rather is a secondary adaptation in Some species."