The DNA content of the salivary gland was measured by the method of Burton in larvae of Chirmmus thummi at four stages of development during the onset of the larval-pupal metamorphosis. The amount of DNA at first increased in the gland, from 0.28 pg/gland at the mid-fourth instar to 0.45 pg/gland in the early prepupa. This increase was immediately followed by a significant decrease in the amount of DNA to 0.37 pg/gland in the late prepupa.
The loss of DNA is considered to be of major interest since it occurred at a time when chromosomal puffing is observed in some regions of the chromosomes. Puffing reflects gene activity. Furthermore, the loss of DNA was preceded in the early prepupa by an increase in the gland's DNase content. The change in this enzyme represents the first detectable chemical change signaling the onset of glandular metamorphosis and suggests that portions of the polytenic chromosomes are the substrate for this hydrolase. This suggestion is substantiated by the constancy of the cell number within the salivary gland and the loss of nuclear volume in individual gland cells. The destruction of the gland appears to be a self-regulated programmed event occurring in the course of normal development and seems to be mediated by the control of the enzyme DNase.