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GENERAL PRINCIPLES OF ZOOLOGY. 



animal pole and here forms a disk of small cells, the rudi- 

 ment of the embryo, or the blastoderm (telolecithal, mero- 

 blastic eggs) (Fig. 99). 

 A 



\ 



FIG. 9> Discoidal cleavage of the egg of a Cephalopod (Lol/go Pealii.) (After Watase.) 



4. Superficial Cleavage. The yolk is collected in the 

 centre of the egg and prevents cleavage ; in consequence 

 of this, only the outer layer of the egg divides into cleavage 

 cells, which, in the form of a continuous superficial layer, 

 enclose the unsegmented central mass (centrolecithal eggs) 

 (Fig. 100). 



A 



B 



FIG. ioo. Superficial cleavage of an insect's egg (Pieris cratcegi). A, division of the cleav- 

 age nucleus ; B, movement of the nuclei to the periphery to form the blastoderm ; C, 

 formation of the blastoderm. (After Bobretzky.) 



Distribution of the Modes of Cleavage. Of the four 

 modes of cleavage mentioned the superficial one has an 

 interest from the point of view of the systematist, since it 

 occurs exclusively in the Arthropods. The other modes 

 of cleavage are distributed as follows : the discoidal has 

 been observed in the majority of the vertebrates and in 

 the most highly organized molluscs, the cuttlefishes, while 



