ABSTRACT
The main physiological function of cleavage is to redress the balance between the size of the nucleus and the volume of cytoplasm with which it is associated. Egg-cells are always very large, as cells go. During the growth period of the oocyte, the nucleus is also large, being distended to form the germinal vesicle. But after fertilisation, the new zygote nucleus is of about normal size for the species in question, and it fmds itself in a cell which is far larger than normal growing cells (although some fully differentiated cells are again very large). During cleavage, the balance is restored. This involves not only a reduction in cell-size by subdivision, but also the formation of new nuclear material to build up the increased number of nuclei. The two most important classes of substance required for these nuclei are the proteins and desoxyribose nucleic acid which together make up the chromosomes. Very little is known of the source of origin of the chromosomal proteins, which cannot easily be isolated from the other proteins of the egg. Technical methods are available for studying the nucleic acids, including the ribose nucleic acid (RNA) which is characteristically present in the cytoplasm as well as the desoxyribose (DNA) compound found in the chromosomes. It was at one time suggested (by Brachet) that in many invertebrates the DNA was formed by conversion of the cytoplasmic RNA (a 'partial synthesis') but this now
seems to be unlikely. It has in fact recently been argued (Zeuthen 1951, Hoff-J0rgensen 1954) that in many forms no net synthesis of DNA takes place during the early part of cleavage, since the cytoplasm of the egg contains stores of this substance which are sufficient to provide for many cleavage nuclei-perhaps a few million in the chick, and a few thousand in the frog, though only about sixteen in the sea-urchin. While this DNA is being incorporated into the nuclei, it is in a state of metabolic activity, since radio-active phosphate is rapidly taken into it (e.g. Villee and Villee 1952); probably also changes are going on in its specrncity, converting it into material capable of acting as genetic determinants, but little is known about this.
