222 



INTERNAL FACTORS 



IV. i 



Isolated i (A D) or \ (AB or CD) larvae have an apical organ 

 and a prototroch complete or semicircular ; they may remain 

 open or close up ; sometimes they gastrulate. 



It is evident that each cell has exactly the same potentialities 

 for division and differentiation when isolated as when in con- 

 nexion with its fellows. Only in the shifting of the cells 

 and closing up of the whole cell-mass is there anything which 

 approaches to total development. 



(1 e 



FIG. 182. Patella. Development of isolated J basal (macromere). 

 a, First ; b, second division ; c, d, 56- and 64-cell stage. In c the 

 position of the 2-group is normal, not in d; e, after 24 hours, showing 

 two secondary trochoblasts (products of 2 11 ) and two feebly ciliated 

 cells (preanal cells ?). (After Wilson.) 



The dependence of this specification of the blastomeres upon 

 definite substances preformed though not prelocalized in the 

 unsegmented ovum is demonstrated by Wilson's experiments on 

 Denlalium. The newly laid egg of Dentallum contains a brick- 

 red yolk-mass surrounded by a thin hyaline ectoplasm ; the latter 

 is thickened a little to form an animal polar area and very con- 

 siderably to form another large area at the vegetative pole. The 

 nucleus is in the centre (Fig. 133). When the sperm enters the 

 nucleus breaks down, and its substance becomes confluent with both 



