58 PRINCIPLES OF EMBRYOLOGY 



seems to be unlikely. It has in fact recently been argued (Zeuthen 195 1, 

 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 miUion in the chick, and a few thousand 

 in the frog, though only about sixteen in the sea-urchin. While tliis 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 specificity, converting 

 it into material capable of acting as genetic determinants, but httle is 

 known about this. 



In the readjustment of the nuclear-cytoplasmic ratio, subdivision of the 

 active protoplasm is of much more importance than cutting up of the 

 inactive yolk, which probably makes no chemical demands on the nucleus. 

 The progress of cleavage is accordingly always profoundly modified by 

 the presence of appreciable quantities of yolk. We fmd, for instance, that 

 in yolky eggs, the fertilised nucleus is displaced from the centre of the 

 egg towards the less yolky end. Moreover, the cleavages begin earher 

 and go on faster in the less yolky parts. And very often the cleavage 

 spindles are orientated so as to He with their axes in the direction of the 

 longest stretch of non-yolky cytoplasm available in the cell — but we 

 shall see that this rule (sometimes known as Balfour's rule) is not the only 

 factor at work in controlling the spindle directions, since these may be 

 definitely orientated even in eggs where there is too httle yolk to make 

 Balfour's mechanism effective. 



The result of these factors is that in eggs with a fair amount of yolk, 

 the cleavages produce more and smaller cells in the less-yolky animal 

 region than in the more yolky vegetative end. Where the store of yolk is 

 very large, the most heavily laden region may not be divided at all during 

 cleavage; in fact, in extreme cases such as reptile and bird eggs, cleavage is 

 confined to a small superficial area near the animal pole, where alone 

 there is any appreciable quantity of cytoplasm (Fig. 4.1). 



We may therefore classify cleavage types as follows: 



Total cleavage. Whole egg divides. 



(i) Equal. In eggs with httle yolk. 



(ii) Unequal. In eggs with rather more yolk. 

 Partial cleavage. Part of the egg remains undivided; in eggs with rather 

 large stores of yolk. 



Superficial cleavage. Cleavage only in a small area of the egg; in ex- 

 tremely yolky eggs, 



