CHAPTER FIVE 

 Cleavade 



Determination of Planes 



Cleavage Laws 



Steps in the Cleavage Process 



Determination of Planes. 



The plane of the first cleavage is determined in a very different 

 manner. The cytoplasm within which the cleavage is initiated is some- 

 what flattened beneath the pigmented cortex of the animal hemi- 

 sphere. The egg nucleus lies within this cytoplasm, and, as the sperm 

 nucleus approaches it, preceded by its divided centrosome, an amphi- 

 aster is set up to divide the chromatic material of the two syngamic 

 nuclei. The cleavage plane therefore is determined by the direction of 

 approach of the sperm nucleus, or rather, the copulation path. It can 

 be assumed that in the undisturbed egg the cytoplasm also has a slight 

 axial distribution, paralleling that of the yolk, with its maximum in 

 the opposite direction to the yolk. The cleavage furrow will form at 

 right angles to the amphiaster, and this coincides exactly with the 

 sperm copulation path. One may conclude, therefore, that under un- 

 disturbed conditions the first cleavage furrow includes (or is de- 

 termined by) the copulation path. 



It should be made clear at this point, however, that the cleavage 

 plane need not necessarily represent the median longitudinal axis 

 (sagittal plane) of the embryo, cutting it into right and left halves. 

 Should this happen it would mean that the penetration and the copula- 

 tion paths were in exactly the same plane and the first cleavage would 

 represent a median sagittal plane of the future embryo. Further, the 

 mere pressure from other eggs in the same clutch can so alter the dis- 

 tribution of cytoplasm that the cleavage spindle may be shifted from 

 its expected position. The median plane of the bilaterally symmetrical 

 embryo may, but does not necessarily, coincide with the plane of egg 

 symmetry. 



To summarize, the penetration point combined with any two points 

 on the egg axis establishes a plane which represents the median sagit- 



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