142 GENERAL PRINCIPLES OF ZOOLOGY 



cleft from each other (fig. 101). As a rule each new plane of cleavage is 

 perpendicular to the preceding. Hence the first three cleavage planes, 

 which cause the division into 2, 4, and 8 parts, are similarly arranged in 

 most animals. Using the globe for comparison, one speaks of a first and 

 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 vegetative, so called because the material of the 

 one is used chiefly for animal organs (nervous system), that of the other 

 for vegetative organs (digestive tract). 



i I ni 



FIG. 101. The equal cleavage of Amphioxus lanceolatus (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 hid- 

 den;) TV, blastula, in optical section; a single layer of cells surrounds the cleavage 

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



Influence of the Yolk upon Segmentation. Different kinds of 

 cleavage are distinguished, the peculiarties of which depend upon two 

 factors: (i) upon the quantity of material (food-yolk) serving for nourish- 

 ment of the egg; (2) upon its arrangement. The food-yolk hinders the 

 division, since it is incapable of active movement, and is only passively 

 divided through the activity of the protoplasm. The more this increases 

 in proportion to the protoplasm, the more slowly does the cleavage process 

 proceed. 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 of the egg is divided, that 

 with much yolk remaining undivided. In this case we have a partial 

 cleavage in comparison with the more primitive mode, the total cleavage; 

 further, eggs which show a partial cleavage are called meroblastic, because 

 only the segmented part of the egg is directly employed in the formation of 

 the embryo (^Xao-ro's), while the undivided main mass serves merely as 

 food-material. Eggs with total cleavage are called Jioloblastic. 



The distribution of the yolk is connected with the position of the 

 nucleus; either this is central and the yolk is concentric around it (cen- 

 trolecithal eggs), or it is pushed, together with the greater part of the pro- 

 toplasm, to one pole of the egg, while at the other pole the yolk predomi- 



