CHAPTER IV 



CLEAVAGE 



I. General features 



In all eggs, fertilisation is followed by a period in which the egg divides 

 into a number of cells. This is known as the period of cleavage. It may be 

 said to last until some other process, besides mere subdivision, begins to 

 become important; the process in question is usually a shifting of regions 

 of the egg relative to one another — a series of movements which, as we 

 shall see, will eventually bring about gastrulation. By the time these 

 movements begin the egg has been extensively cut up into smaller cells, 

 and from this time onwards the zygote is commonly referred to as an 

 'embryo' instead of, loosely, as an 'egg'. As we saw in Chapter I the 

 cleavage cells at the beginning of gastrulation are arranged as a 'blastula', 

 which in its most typical form is a hollow sphere. We shall find, however, 

 that the course taken by the cleavages in many groups is such that the 

 blastula is considerably modified from this pattern. 



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 biown 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 sug- 

 gested (by Brachet) that in many invertebrates the DNA was formed by 

 conversion of the cytoplasmic RNA (a 'partial synthesis') but this now 



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