88 CLEAVAGE 



tal plane of the future embryo. The copulation (syngamic) path of the 

 sperm nucleus to the egg nucleus determines the first cleavage plane. 

 Should the penetration and copulation paths be in a direct line to the 

 egg nucleus, the first cleavage would bisect the future embryo ( and the 

 gray crescent) into right and left halves. There is a natural tendency 

 for the coinciding of the gravitational plane, the sperm entrance point, 

 the sperm penetration path, the median plane of the egg, the median 

 plane of the embryo, and the plane of the first cleavage. But this is not 

 essential to the production of a normal embryo. 



Cleavage Laws. 



There are four major cleavage laws, known by the names of those 

 who first emphasized their importance. 



1. Pfluger's Law, The spindle elongates in the direction of least 

 resistance. 



2. Balfour's Law. The rate of cleavage tends to be governed by 

 the inverse ratio of the amount of yolk present, in holoblastic cleav- 

 age. The yolk tends to impede division of both the nucleus and the 

 cytoplasm. 



3. Sack's Law. Cells tend to divide into equal parts and each new 

 plane of division tends to bisect the previous plane at right angles. 



4. Hertwig's Law. The nucleus and its spindle generally are found 

 in the center of the active protoplasm and the axis of any division 

 spindle lies in the longest axis of the protoplasmic mass. Divisions tend 

 to cut the protoplasmic masses at right angles to their axes. 



Steps in the Cleavage Process. 



The frog's egg is telolecithal, meaning that there is a large amount 

 of yolk concentrated at one pole, opposite to the concentration of 

 cytoplasm and the location of the nucleus. The cleavages are holo- 

 blastic (i.e., total), and, after the second cleavage, they are unequal. 

 They eventually cut through the entire egg from the surface inward. 

 About 2Vi hours after the egg is fertilized there appears a very short 

 inverted fold near the center of the pigmented and slightly depressed 

 animal hemisphere cortex. This depression is extended in both direc- 

 tions and in the pigmented surface coat there appear lines radiating 

 outwardly from the deepening first cleavage furrow. It appears as 

 though some internal force is drawing this cortical region (surface) 

 of the egg toward its center, and the apparent tension lines may be the 



