THE FUNCTION OF THE CENTROSOME. 



205 



(Fig. i). In the subsequent cleavages, however, the differences 

 are very pronounced, and the cells may be classed in four cate- 

 gories, according to their peculiarities with reference to the rate 

 of division: (i) cells which divide much more rapidly than any 

 of the others, e.g., those marked x and m in the diagram, which 

 form respectively the ectoderm and the mesoderm of the trunk ; 

 (2) cells which divide more slowly but continuously, e.g., ec, 

 which form part of the general ectoderm of the head; (3) cells 

 which cease dividing for a con- 

 siderable period, but later re- 

 sume their karyokinetic activity 

 and undergo rapid segmentation, 

 e.g., the entoderm cells, stippled 

 in the diagram ; (4) cells which 

 never divide nor show any signs 

 of karyokinetic activity, e.g., 

 those marked/, and these soon 

 develop motile cilia and consti- 

 tute the primary prototroch, 

 a larval organ of locomotion. 

 In the latter category may be 

 placed also the cells marked /', 

 for three of the four cells de- 

 rived from the division of each of these also cease dividing, be- 

 come ciliated, and contribute to the formation of the prototroch. 



What are the influences which compel the prototroch cells 

 {/) to cease dividing, while the adjacent cells (x) divide with 

 extraordinary rapidity ? Why do the cells of the general 

 ectoderm (ec) divide at an ordinary rate, while karyokinesis in 

 the neighboring entoderm cells is temporarily suspended ? 



Many more or less plausible suggestions as to the nature of 

 the extrinsic conditions which determine the behavior of cleav- 

 age cells have been advanced by students of developmental 

 mechanics, the gist of which is tersely expressed by Driesch: 

 The prospective significance of a blastomere is a function of its 

 position, the effects of mutual pressure, of surface tension, of 

 gravity, etc., varying according to the position of the blastomeres 

 in the egg, and according to the position of the egg itself. 



FIG. i . Side view of the egg of Amphjtrite 

 at about the 64-cell stage. The heavy lines 

 indicate areas of differentiation; /, primary 

 prototroch-cells ; /', secondary prototroch- 

 cells ; x, somatic plate : ec, general ectoderm ; 

 m, mesoderm ; stippled cells, entoderm. 



