Chapter V 



CLEAVAGE AND DIFFERENTIATION 



§1 

 The most obvious visible change during the first phase of develop- 

 ment of the fertilised egg is its cleavage into a number of separate 

 cells. We must now ask whether other equally important but less 

 obvious changes may not be taking place at the same time, and en- 

 quire into the relation between cleavage and the processes leading 

 to morphological differentiation. 



The pattern of cleavage is normally oriented in relation to the 

 existing major axis of the egg, e.g. the first two cleavage planes are, 

 in all known cases except one, meridional: the exception is pro- 

 vided by the Nematodes, where the plane of the first cleavage is 

 still oriented with reference to the axis, but at right angles to it, 

 and therefore latitudinally. In Cephalopods and Ascidians, the 

 cleavage pattern is oriented with reference to the secondary axis 

 of bilateral symmetry as well. 



The orientation of cleavage-pattern can, however, be modified. 

 It may be modified in relation to a new, induced, axis of polarity. 

 For instance, in the sea-urchin Lytechinus and the star-fish Patina, 

 cut fragments of the unfertilised egg, subsequently fertilised, al- 

 ways have the first two cleavage planes at right angles to the cut 

 surface, which, as we shall see later (p. 313), has established a new 

 polarity.^ 



The cleavage-pattern may also be modified by mechanical means, 

 e.g. by a restratification of the egg-contents by the use of the centri- 

 fuge (see p. 218). In the sea-urchin Arbacia, for instance, the first 

 two cleavages are perpendicular to the stratification, whatever its 

 relation to the original axis.^ Or the cleavage-pattern maybe altered 

 by forcing eggs to undergo cleavage while compressed between 

 glass plates. The orientation of the division spindles in these cases 

 is governed by the principle known as " Hertwig's rule ", which lays 

 down that at mitosis the spindle will form with its long axis in the 



^ Taylor and Tennent, 1924; Taylor, Tennent and Whitaker, 1925; Taylor 

 and Whitaker, 1926. 2 Morgan and Lyon, 1907. 



6-2 



