292 CLEAVAGE (SEGMENTATION) AND BLASTULATION 



plane nearest the animal pole divides each of the eight micromeres into an 

 upper and a lower micromere, while the plane which furrows the eight macro- 

 meres divides each into upper and lower macromeres. Thirty-two cells are, 

 thus, the result of the fifth cleavage planes. The lowest of the macromeres 

 are larger and laden with yolk material (fig. 140L). The sixth cleavage planes 

 are synchronous and approximately meridional in direction in all of the 32 

 cells, resulting in 64 cells (fig. MOM). The blastocoelic cavity is a con- 

 spicuous area in the center of this cell mass and is filled with a jelly-like 

 substance (fig. 140N). Study also figure 167. 



When the eighth cleavage furrows occur, the blastocoel contained within 

 the developing blastula is large (fig. 140?). As the blastula continues to 

 enlarge, the blastocoel increases in size, and the contained jelly-like substance 

 assumes a more fluid condition (fig. 140O-S). The fully formed blastula is 

 piriform or "pear-shaped" (fig. MOT). (See Conklin, '32.) 



The cleavage pattern of the urochordate. Styela partita, is somewhat similar 

 to that of Amphioxus, but considerable irregularity may exist after the first 

 three or four cleavages. In Styela the ooplasm of the egg contains differently 

 pigmented materials, and yellow and gray crescentic areas are visible at the 

 time of the first cleavage. (See fig. 132.) These different cytoplasmic areas 

 give origin to cells which have a definite and particular history in the embryo. 

 Observations devoted to the tracing of such cell histories are grouped under 

 the heading of "cell lineage." Cell-lineage observations are more easily made 

 in the eggs of certain species because of definitely appearing cytoplasmic 

 areas, where colored pigments or other peculiarities associated with various 

 areas of the egg make possible a ready determination of subsequent cell 

 histories. The general organization of the egg of Amphioxus, regardless of the 

 fact that its cytoplasmic stuffs do not have the pigmentation possessed by the 

 egg of Styela, appears similar to that of the latter (cf. figs. MOA; 167A). 

 (See Conklin, '32.) 



b. Frog (Rana pipiens and R. sylvatica) 



The egg of the frog is telolecithal with a much larger quantity of yolk than 

 is found in the egg of Amphioxus. The pattern of cleavage in the frog, there- 

 fore, is somewhat less ideally holoblastic than that of Amphioxus. 



The first cleavage plane of the frog's egg is meridional (figs. MIC; M2A-C). 

 It occurs at about three to three and one-half hours after fertilization at ordi- 

 nary room temperature in Rana pipiens. It begins at the animal pole and 

 travels downward through the nutritive or vegetal pole substance, bisecting 

 both poles of the egg. In the majority of eggs, it bisects the gray crescent. (See 

 p. 287.) The second cleavage plane divides each of the first two blastomeres 

 into two equal blastomeres; its plane of cleavage is similar to the first cleavage 

 plane but is oriented at right angles to the first plane (figs. MID; 142D-E). 

 The upper, animal pole end of each of the four blastomeres contains most 



