56 



EMBRYOLOGY. 



name morula or mulberry-sphere, because the cells protrude as small 

 elevations at its surface. 



During the second and third stages of cleavage there is easily 

 recognisable a rigidly observed order in the direction which the planes 

 of cleavage sustain to each other. The second plane of cleavage always 

 halves the first and cuts it perpendicularly ; the third plane, again, is 

 perpendicular to the first two, and passes through the middle of the 

 axis formed by their intersection. If one regards the ends of this 

 axis as the poles of the egg, the first two planes of division may be 

 designated as meridional, the third as equatorial. 



This uniformity is caused by the mutual relation which subsists 

 between nucleus and protoplasm, in which connection the two follow- 

 ing laws are to be noted : (1) The plane of division always cuts the 

 X. axis of the spindle perpendicularly at its centre. (2) The position of 



Fig. 30. Various stages of the process of cleavage, after GEOENBAUR. 



the axis of the nuclear spindle in turn depends on the form and differ- 

 entiation of the protoplasmic body which envelops it, and in such a 

 manner that the two poles of the nucleus take the direction of the greatest 

 protoplasmic masses. Thus, for example, in a sphere in which the 

 protoplasm is uniformly distributed, the centrally situated spindle 

 may come to lie in any radius ; but in an ovoid protoplasmic body, 

 only in the longest diameter. In a circular protoplasmic disc the 

 nuclear axis lies parallel to its surface in any diameter whatever of 

 the circle, but in an oval disc, as before, in the longest diameter 

 only. 



Let us return now, after these general remarks, to the case under 

 consideration. Each daughter-cell forms at the close of the first seg- 

 mentation a hemisphere. According to the rule, the daughter-spindle 

 cannot assume a position perpendicular to the flat surface of the 

 hemisphere, but must lie parallel to it, so that a division into two 

 quadrants must result. At the next segmentation the axis of the 

 spindle must coincide with the long axis of the quadrant, whereby 

 this becomes divided into two octants. 



