ABSTRACT
The structure, distribution, cytochemical nature and functional significance of mitochondria have been studied during the embryogenesis of the slug Arion, from the ovum to the postgastrula stage.
Mitochondria seem to undergo progressive but profound changes in form and fine structure during early development and their distribution conforms to a gradient pattern, which more or less resembles that of the sea urchin.
The most significant event in cyto-differentiation is the appearance of an extraordinary vegetal aggregation of mitochondria at the 8-cell stage which heralds a vegetalizing influence and undoubtedly has a precise morphogenetic function. This vegetal aggregation becomes involved in a remarkable series of morphogenetic movements and its mitochondria are later incorporated into the cells that differentiate into mesoderm and endoderm. Thus, apart from their energy-linked functions, mitochondria seem to play a dramatic directive role in development; mitochondrial segregation, in particular, seems to be the important factor in differentiation.
Certain aspects of their distribution and enzymic composition are discussed in relation to their normal metabolic functions.
The significance of mitochondrial RNA and DNA with reference to protein synthesis and self-duplication of mitochondria, and the possible roles played by mitochondria in inductive phenomena are also discussed in the light of recent work.
