THE CELL ORIGIN OF THE PROTOTROCH. II5 



Fig. 3. — Loxodromic curve (" left-handed spiral ") described 

 about a sphere, showing the general direction of the 

 cleavage furrows which cut the cells of the 2-, S-, and 

 32-cell stages. 



size of the resulting blastomeres, yet the irregularity is not so 

 great as to obscure the rhythm of cleavage, which expresses 

 itself in the following 

 manner : 



Each of the two cells 

 divides, so that we have 

 a 4-cell stage ; then each 

 of these cells divides, 

 and so on, the number 

 of cells increasing in 

 geometrical progression 

 until the egg consists 

 of sixty-four cells. A 

 distinct rhythm is ob- 

 served also in the direc- 

 tion of cleavage, for the 

 cleavage planes lie ob- 

 liquely across the meridians of the Q^g. The cells in the 2-, 8-, 

 and 32-cell stages are divided by cleavage furrows which take 

 the general direction of the loxodromic curve in Fig. 3,^ which 

 may be rather loosely but conveniently defined as a left-handed 



spiral described around a 

 sphere from pole to pole. 

 The cells of alternate gen- 

 erations, namely, of the 4- 

 and i6-cell stages, are cut 

 by furrows which have an 

 opposite direction, as rep- 

 resented in Fig. 4. These 

 curves may be called, for 

 convenience, left and right 

 respectively, following 

 the conventional nomen- 



FiG. 4— Loxodromic curve ("right-handed spiral"), claturC for thc thrCads OU 

 having the general direction of the furrows which cut 

 the 4- and i6-cell stages and many of those of the 64- 3- SCrCW. 



cell stage. Xhc rhythm of division 



1 In the diagram this curve cuts the equator at one point only. The curve 

 will, of course, cut the equator at any point, if rotated. 



