CLEAVAGE AND THE FORMATION OF THE BLASTODERM. 



265 



unlike a wandering amoeboid cell. Since, however, all these proto- 

 plasmic islands are connected by a fine network of processes, the 

 whole egg constitutes a syncytium, the yolk being contained in the 

 meshes. Nevertheless, although these formative centres are not yet 

 distinctly marked off they are already called (though inaccurately) 

 cleavage-cells. 



In later stages these " cleavage-cells " shift somewhat nearer the 

 surface of the egg, and become arranged to form a sphere (Fig. 130 A, 

 131 B) almost parallel to the latter. In sections of the egg they 

 therefore appear arranged as a circle (Fig. 130 ^4). Gradually, during 

 further processes of division, they reach the surface of the egg and 

 fuse with the peripheral protoplasm found there (Figs. 130 B and 



Fig. 131.— The formation of the blastoderm in HydropMlus (after Heider). b, developed blas- 

 toderm ; d, food-yolk ; /, so-called cleavage-cells ; fc, peripheral protoplasm; z, yolk-cells. 



131 C). Division into separate cell- territories corresponding to the 

 cleavage-nuclei now takes place (Figs. 130 C and 131 D) by the 

 appearance of furrows that press in from the surface, and gradually 

 traverse the whole of the peripheral protoplasm (Fig. 130 D). 

 After the surface of the egg has been covered in this way with an 

 epithelium (blastoderm), there follows, in many Insects (Chironomus, 

 Musca, H>jdro])hilus), the separation of the so-called inner peripheral 

 protoplasm (Fig. 130 D, i), i.e., of a layer of protoplasm containing 

 coarse granules which develops between the blastoderm and the 

 central yolk-mass. By taking up this layer of protoplasm, the 

 blastoderm -cells increase in height, and now form a cubical or 



