vm] CLEAVAGE PLANES 119 



which will form the right and left halves of the embryo, 

 although experiment shows that in eggs of this kind each of 

 these blastomeres has in it the potentiality of producing a 

 whole embryo if they are artificially separated. What, then, 

 determines the position of the first cleavage plane? The 

 plane of cell-division is always perpendicular to the axis of 

 the mitotic spindle, and this in turn depends on the position 

 of the centrosomes. In an egg which is truly equipotential 

 there would appear to be only one factor which could 

 determine what position the centrosomes shall take up; 

 this factor is the entrance path of the spermatozoon. As 

 the sperm nucleus travels towards the egg nucleus the 

 centrosome derived from the accompanying middle-piece 

 divides and the daughter centrosomes diverge in a line at 

 right angles to the entrance path ; since, therefore, the plane 

 of cytoplasmic division is at right angles to the line joining 

 the centrosomes it must coincide with the entrance path of 

 the sperm nucleus, if this is straight, and even if it is curved 

 the division plane has a definite relation to it. In the 

 majority of eggs there is a definite polarity, but all meridians 

 running through the two poles are alike, and in eggs of this 

 type, as was shown by Roux in the Frog, the first cleavage 

 plane runs through the poles, but its meridian is usually 

 determined by the path of entrance of the spermatozoon. 

 Subsequent investigators (BRACKET and others) found that 

 in the Frog the first cleavage plane does not always coincide 

 with the sagittal plane of the embryo; in about 70 per cent, 

 of the eggs it does so, but in the remainder it is either at 

 right angles to it or inclined to it at a varying angle, often 

 approaching 45. Although, however, the first cleavage 

 plane is variable in position, the entrance path of the 

 spermatozoon nevertheless always determines the plane of 

 bilateral symmetry of the embryo, for the substances of 



