THE EARLY DEVELOPMENT OF THE FROG 89 



during cleavage and not later, although there seems to be no 

 definite causal relation between the direction of the first cleav- 

 age plane itself and the symmetry of the embryo, although 

 there may be certain constancies in this relation. 



We may finally mention briefly the relation between the 

 symmetry of the egg (and therefore in general of the embryo) 

 and the symmetry of cleavage, particularly the plane of the 

 first cleavage furrow. The position of the cleavage plane is of 

 course the direct result of the position of the cleavage spindle; 

 it is therefore the position of this latter which is essential. The 

 spindle always lies at right angles to the egg axis, in agreement 

 with the law of Hertwig. In such a plane the position of the 

 spindle is readily influenced by at least one external factor, 

 namely pressure, in such a way that it tends to lie at right 

 angles to the direction of the pressure, and the resulting cleav- 

 age would therefore occur in the direction of the pressure. In 

 a large mass of eggs this factor is probably one of considerable 

 importance, especially in affecting the direction of the first 

 cleavage in those eggs in the interior of the mass. The relation 

 between the direction of pressure and the symmetry of the egg 

 is purely accidental and consequently we find much variation 

 in the relation of these two planes. 



When the egg is not subject to pressure there is a fairly 

 marked tendency for the spindle to lie either transversely to 

 the plane of symmetry or in that plane (Fig. 27, C) . The 

 symmetrical structure of the egg is fairly well established by 

 the time the spindle forms, and there are only these two 

 positions which the spindle can occupy and yet retain sym- 

 metrical relations to the internal structure of' the egg. The 

 former relation, in which the plane of the resulting cleavage 

 would coincide with the plane of egg symmetry, is the more 

 frequent and in approximately 25 per cent, of eggs the first 

 furrow deviates less than five degrees, plus or minus, from this 

 plane. The second relation, placing the first furrow at right 

 angles to the median plane (within five degrees ; plus or minus) 

 is found in something like 10 per cent, of eggs (Jenkinson). 



But the position of the spindle seems to be influenced quite 



