GENERAL EMBRYOLOGY. 151 



3. Cleavage Process. 



Arrangement of the Cleavage Planes. The fertilized egg-cell 

 divides in rapid succession into 2, 4, 8, 16, etc., cells, which 

 become continually smaller, since the mass of the egg does not 

 increase. The cells are called cleavage spheres, or blastomeres, 

 the whole process the cleavage process, or segmentation, 

 because, at each division, furrows arise on the surface which 

 continue to penetrate more deeply (fig. 93). As a rule each 



FIG. 96. The equal cleavage of Amphioxws lanceolattis. (After Hatschek.) I, division 

 into two (formation of the first meridional furrow); II, division into four (second 

 meridional furrow) forming four cleavage spheres (fourth is hidden); III, division 

 into eight (equatorial furrow; the seventh and eighth cleavage spheres hidden) ; 

 IV, blastula in optical section. A single layer of cells surrounds the cleavage 

 cavity. In I, II, III, a polar body is shown. 



new plane of cleavage is as nearly as possible perpendicular to the 

 preceding. Hence the first three cleavage planes, which cause the 

 division into 2, 4, and 8 parts, are similarly arranged in almost all 

 animals. Using the globe as a basis for comparison, one speaks 

 of a first and a second meridional furrow (I, II), and calls the third 

 the equatorial furrow (III). The intersections of the two meridi- 

 onal furrows form the poles of the egg, the animal and the 

 vegetative, so called because the material of the one is used chiefly 

 for animal organs (nervous system), the material of the other for 

 vegetative organs (digestive tract). 



Influence of the Yolk upon Segmentation. Different kinds of 

 cleavage processes are distinguished, the peculiarities of which 

 depend upon two factors: (1) upon the quantity of material, food- 

 yolk, serving for nourishment of the egg; (2) upon the arrange- 

 ment of this. The food-yolk hinders the division, since it is a 

 material which is incapable of active movement, and is only 

 passively divided through the activity of the protoplasm in the 

 cleavage cells. The more the mass of this increases in proportion 

 to the protoplasm, the more slowly does the cleavage process pro- 

 ceed. Finally there comes a point where the resistance of the 

 yolk becomes so great that the protoplasm is no longer able to 

 carry out the work completely; then only the protoplasmic part 



