244 Ed?fiiind B. inisoH. 



duce a pair of typical prototrochal cells. Isolated second prod- 

 ucts (i"-^"S i"'^", etc.) undergo no further division, but different- 

 iate singly into typical prototrochal cells. 



5. Isolated i^ cells produce embryos bearing at the anterior 

 end an apical organ, at the other two secondary trochoblasts. 



6. Isolated products of the i^ cells differentiate into typical 

 sensory cells of the apical organ, into secondary trochoblasts, and 

 into less differentiated ectoblast cells. 



7. Isolated >^-macromeres produce closed embryos that gas- 

 trulate and bear at one end one or two secondary trochoblasts, 

 and at some other point a small group of feebly ciliated cells, 

 probably representing the pre-anal ciliated cells of the normal 

 larva. 



8. Isolated yV-macromeres produce closed embryos that 

 gastrulate, bear no trochoblasts, but have feebly ciliated cells, as 

 in 7. 



9. Isolated cells of the second quartet produce closed ecto- 

 blastic embryos bearing one or two secondary trochoblasts, and 

 one or two feebly ciliated cells, that probably also represent part 

 of the pre-anal tract. These embryos do not gastrulate, but may 

 form mesenchyme-like cells. 



10. Isolated >4-blastomeres produce embryos that gastrulate, 

 produce four primary trochoblasts, at least two secondary ones, 

 and an apical organ. 



11. Isolated ^-blastomeres produce, in Patella, larvae bear- 

 ing an apical organ, and a prototroch, either open or closed, ac- 

 cording to the mode of cleavage. In DentaUiim only the CD 

 half produces an apical organ and a post-trochal region, and 

 probably only this half produces caelomesoblast. 



12. The development of both Patella and Dentaliuvi is essen- 

 tially a mosaic-work of self-differentiating cells. 



v. DISCUSSION OF RESULTS. 



The experimental results brought forward in this paper and the 

 preceding one seem to me to establish definitely the principle of 

 mosaic development in the case of the mollusks Dentalium and Pa- 



