Printed in Great Britain © The Company of Biologists Limited 1986 THE OCCURRENCE OF LAMPBRUSH CHROMOSOMES

Transmission electron microscopy has been used to study the chromosomes found in the early diplotene oocytes of Xenopus laevis. Chromosomes from 40 /Urn early diplotene oocytes were found to possess a normal lampbrush chromosome morphology. The contour length of the loops found on these chromosomes was measured, the mean value determined and compared with that for lampbrush chromosomes found on 300/xm mid-diplotene (Dumont stage II) oocytes. The mean contour length of the loops from 40 fim oocytes (12-33 ± 6-5 jim) was longer than that for the 300 ^m oocytes (7-897 ± 5-22/im). Analysis of variance showed these two values to be significantly different (P<0-01). The mean loop density (number of loops per 25/im chromosome axis) was also determined for the two size classes of oocytes, being 9-33 ± 2-05 per 25 fjm of chromosome axis for the 40 /im size class and 11-2 ± 2-435 per 25 ̂ m of chromosome axis for the 300 /im size class. Analysis of variance showed these values to be significantly different (P< 0-01). The results of the present study demonstrate that the lampbrush chromosomes found in 40 fim early diplotene oocytes possess a greater loop contour length and a lower density of loops than those found in 300 fim, mid-diplotene oocytes. Possible reasons for these observations are discussed