460 BERTRAM G. SMITH 



Stage 2: (figs. 62 to 61^ and 205). The second cleavage furrow 

 makes its appearance about six hours after the first, which by 

 this time extends nearly to the equator of the egg. In the stage 

 represented in figure 205, the first cleavage furrow has just reached 

 the equator. The second cleavage furrow usually becomes super- 

 ficially complete in Stage 4 (fourth cleavage) , quite uniformly 

 meeting the first cleavage furrow at right angles at the vegetal 

 pole. 



The earliest indication of the second cleavage furrow is usually 

 a roughness in the region of the animal pole where the second 

 groove is to intersect the first. The occurrence of 'Faltenkranzen' 

 - — a quivering of the surface with the formation of fine radiating 

 or parallel wrinkles, which extend outward from the cleavage 

 furrow — is quite marked at the time of the initiation of the second 

 cleavage furrow. The cause of the formation of similar wrinkles 

 in the frog's egg has been investigated by Charles B. Wilson 

 ('96). For some time after its first appearance the second furrow 

 is much broader, though of course shallower, than the first. 



The second cleavage furrows usually depart from the same 

 point as the first furrow, and proceed vertically, forming a single 

 straight line at right angles to the first furrow (figs. 62 and 205). 

 Occasionally the points of departure of the second furrows do 

 not coincide, as shown in figures 63 and 64. The condition 

 shown in figure 63 is rarely observed, and is transitional to that 

 shown in figure 64, which is quite frequent. That portion of the 

 first cleavage furrow lying between the points of departure of 

 the second furrows may be called the polar furrow. As shown 

 by the individual histories of a large number of eggs, in all cases 

 in which a polar furrow is present the points of departure of the 

 second cleavage furrows are separate from the beginning; the 

 polar furrow at first exists as a part of the straight first cleavage 

 furrow, but later becomes oblique through the shifting of cells. 

 In no case in which the second cleavage furrows have their origin 

 from the same point in the first cleavage furrow, has there ever 

 been observed any shifting of cells during this or the following 

 stage, of such a nature as to produce a polar furrow. 



