CLEAVAGE AND DEVELOPMENTAL PATTERN 575 



comes the dorsiventral axis of the embryo but is independent of the axis 

 indicated by polar-body position. If these observations are correct, or- 

 ganismic pattern appears to be determined by cleavage pattern or by the 

 same factor that determines cleavage pattern ; and if a polarity is deter- 

 mined in the ovary or by position of polar body-formation, it may be ob- 

 literated by the polarity of the cleavage pattern, which is, or may be, de- 

 termined by pressure. Development of centrifuged eggs seems to support 

 this conclusion (p. 586). There is, however, no evidence of a polar differen- 

 tial in earlier cleavage. The first two cleavages divide the egg into four al- 

 most equal quadrants, with polar cross-furrows, as in spiral cleavages, de- 

 termining a cleavage axis (Fig. 181, A), which supposedly becomes the 



ABC 



Fig. 181, yl-C— Cleavage stages of Cyc/oi'5. A, four-cell stage; B, sixteen-cell stage; C, later 

 stage, showing entoderm cell, E, and primitive germ cell, G, which are formed by division of 

 Z)'"^ of sixteen-cell stage shown in B; heavy line in C indicates boundaries of quadrants of 

 four-cell stage (after Fuchs, 19 14). 



dorsiventral axis of the animal, but dorsal and ventral are distinguishable 

 only in later stages. The first cleavage plane is said to coincide approxi- 

 mately with the median plane of the embryo. The third cleavage is equa- 

 torial with respect to the cleavage axis determined by the first two cleav- 

 ages and is approximately equal; the fourth is meridional or apparently 

 sHghtly oblique (Fig. 181, 5). Entoderm develops from a single cell of the 

 fifth cleavage generation, and its sister cell is the primitive germ cell; both 

 lie at the ventral pole and approximately in the future median plane {E, 

 G, of Fig. 181, C); and cells immediately surrounding them give rise to 

 mesoderm, which is invaginated with entoderm and germ cell. This cell 

 lineage gives no information as to origin or character of organismic de- 

 velopmental pattern. If it is true that cleavage pattern is determined by 

 shape of the egg and that the cleavage axis becomes the dorsiventral axis, 

 it apparently follows that axiate organismic pattern is independent of any 

 pre-existing pattern or egg organization and originates from purely for- 



