1 82 GENERAL PRINCIPLES OF ZOOLOGY. 



3. Cleavage Process. 



Arrangement of the Cleavage Planes. The fertil- 

 ized egg-cell divides in rapid succession into 2, 4, 8, 

 1 6, etc., cells, which become continually smaller, since 

 the mass of the egg undergoes no increase. The cells 

 are called cleavage spheres, the whole process the cleav- 

 age process, because, at each division, furrows arise on the 

 surface which continue to penetrate more deeply (Fig. 

 93)- I n general the rule holds that each new plane of 



I I ffl 



FIG. 93. The equal cleavage of A mphioxus lanceolittus (After Hatschek.) I, division 

 into two (formation of the first meridional furrow i ; II. division into four (second merid- 

 ional furrow) forming four cleavage spheres (third is hidden); III, division into eight 

 (equatorial furrow; the eighth cleavage sphere is hidden); IV, blastula in optical sec- 

 tion. A single layer of cells surrounds the cleavage cavity. In I, II, III, a little body 

 (the polar body) is shown. 



cleavage is as nearly as possible perpendicular to the pre- 

 ceding. 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 com- 

 parison, 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 meridional furrows 

 form the poles of the egg, the animal and the vegetal, so 

 called because the material of the one is used chiefly for 

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

 for vegetative organs (digestive tract). 



Influence of the Yolk upon the Cleavage Process. 

 In embryology there are distinguished different kinds of 

 cleavage processes, whose differences depend upon two 

 factors: (i) upon the quantity of material, food-yolk, serv- 

 ing for nourishment of the egg; (2) upon the arrangement 

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

 material which is not capable of active movement, and is 



