At the first symptoms of organisation of the silkgland in the embryo, mitoses stop and nuclei start to grow. Through autoradiographic studies, performed after sequenced labeling with [3H] and [14C] thymidine, the durations of the different phases of DNA synthesis cycles (T = 42 to 48 h, S = 22 to 25 h, G = 20 to 23 h) are established. These durations are in fact identical during the second and the third instar, and the same, whatever region is concerned: posterior, middle or anterior parts. A model has been established to account for the distribution of the S phases during the second and third instars. The DNA synthesis in all nuclei of a given region has been followed during the first four instars by labelling with [3H]thymidine. The activity goes through maxima and minima, depending on the percent of nuclei synthesizing DNA and their synchronism, both being characteristic of each region; long resting periods are observed during the molting stages of the first three instars in the middle and the anterior parts. The coincidence is obvious between the maxima and minima and respectively the S and G phases of the model. DNA assays agree with the distribution of cycles established by autoradiography if one admits that each cycle does lead to a doubling of the amount of DNA. The overall DNA synthesis from the diploid value is estimated to correspond to 18-19 endomitotic cycles in the nucleic of the posterior part, 19-20 cycles of those of the middle part and 13 in those of the anterior part. The analysis of chromosome structures, by squashing the nuclear content, shows the existence of a complete endomitotic cycle: the doubling of chromosomes is associated with condensed structures, alternating with a decondensed state of chromatin, responsible for the DNA synthesis. The female heterochromatin undergoes a restricted morphological cycle delayed with respect to bulk chromatin. It is characterized by a late DNA duplication and by non dispersed daughter chromosomes. Some of these aspects are, to a lesser extent, reproduced in groups of autosomic chromosomes. © 1979 Masson, Paris.
