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 vi 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, 

 €.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 

 {p) 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 ftinctio7i of its 

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

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

 in the ^^^, and according to the position of the Q^g itself. 



Fig. I. — Side view of the egg of Atnphitrite 

 at about the 64-cell stage. The heavy lines 

 indicate areas of differentiation; /, primary 

 prototroch-cells ; /', secondary prototroch- 

 cells; jr, somatic plate ; ff. general ectoderm; 

 lit, mesoderm ; stippled cells, entoderm. 



