V THE DEVELOPMENT OF THE FROG 91 



rarely it lies at right angles to it, although it may occur in 

 almost any intermediate position. 



The second cleavage appears about three quarters of an 

 hour after the first ; the furrow extends gradually from the 

 animal to the vegetal pole, at right angles to the first furrow, 

 and divides the egg into four cells. The first and second 

 cleavage planes stand in a tolerably constant relation to the 

 axes of the body of the embryo, the first cleavage plane 

 marking the median or sagittal plane of the future animal ; 

 but this is a rule not without exceptions. 



The third cleavage furrow comes in a little above the 

 equator of the egg and at right angles to the other two ; the 

 four upper cells cut off by this division are a little smaller 

 than the lower four. At the next cleavage the furrows run 

 nearly vertically and hence at right angles to the third cleav- 

 age plane. Sometimes the furrows meet at the animal pole, 

 but more frequently they cut through the first or second 

 cleavage furrows, producing thus a bilateral arrangement of 

 the cells. The fourth cleavage furrows are subject to more 

 variation at the vegetal pole of the egg, and the subsequent 

 divisions soon become so irregular that it is impossible to 

 trace out any plan of procedure. A fifth cleavage occurs 

 typically parallel to the third, appearing first in the upper 

 hemisphere and then in the lower. The cleavages thus far 

 v^'viaUy follow the rule that each cleavage plane comes in at 

 right curgi'e-; to the previous cleavage plane. Deviations 

 from the typical method of cleavage are apparently of little 

 moment, even in the first few divisions, as such abnormally 

 dividing eggs may nevertheless produce perfect embryos. 



Cleavage takes place more rapidly in the dark or animal 

 pole, since at that place the protoplasm is most dense. Yolk, 

 which is most abundant in the white or vegetative side of the 

 egg, delays cell division, and we find in the later stages of 



