Abstract
Growth and proliferation during embryonic stage of the posterior silk gland of silkworm, Bombyx mori., have been studied electron microscopically. Growth of the gland could be described subdividing into the following three phases.
(1) The first phase (blastokinesis 24 hours).
This is the latent phase during which the growth in the next proliferation phase is to be prepared. Electron microscopical observations on the posterior silk gland cells have shown that, in this stage, there exist a number of free ribosomes in the cytoplasm. On the contrary, only a small number of endoplasmic reticulum are observable in this phase.
Golgi apparatus is very poorly developed and no Golgi vacuole is found.
(2) The second or proliferations phase (flastokinesis 48-96 hours)
The most striking change is observed in the rough endo plasmic reticulum: The rough endoplasmic reticulum starts to proliferate rapidly. The Golgi vacuoles are still few in number.
(3) The third phase or the stage of fibroin biosynthesis (blastokinesis 120 hours-just before hatchiability). The cytoplasm is mostly filled with tubular or vesicular endoplasmic reticulum, and the intracisternal spaces are now markedly distended. Golgi vacuoles, golgi vesicles, and free vacuoles (fibroin globules) are also quite numerous.
It is concluded that, in the early stage after the blastokinesis, the cellu lar structures necessary for the biosynthesis of fibroin are rapidly formed, while in the later stage the biosynthesis of fibroin proceeds at a maximum rate. These observations suggest that the synthesis and secretion of fibroin in the embryonal posterior silk gland are very similar to those in the fifth larval instar.
