Experimental Studies on Germinal Localization. 245 



tella, and to place the study of cell-lineage on a new and firmer 

 basis. Clearly as the exquisite adjustment between the cleavage- 

 process and the operations of morphogenesis has been revealed 

 by the descriptive-comparative study of cell-lineage, it appears in 

 still stronger relief in the light of the experimental proof that 

 the cleavage-pattern, as a whole and in detail, is the visible ex- 

 presson of an actual distribution of specific morphogenic factors 

 among the cells. 



Although Crampton's initial, and hitherto almost unique, ex- 

 periments on this type of development had led' to the expectation 

 that some evidence of cell-specification and self-differentiation 

 would be found, I confess that I was not prepared to find that evi- 

 dence so circumstantial and consistent. The evidence in Patella 

 that the cleavage-cells are definitely specified from the time of 

 their first formation, and. that they undergo self-differentiation 

 without essential modification through their relation to the other 

 cells, is demonstrative in the case of the cells of the first quartet, at 

 least as far as the i6-cell stage, as shown by the development of 

 isolated entire micromeres at the 8-cell stage, and of their prod- 

 ucts i^ and i^ at the i6-cell stage. It is no less demonstrative in 

 the case of the products of the primary trochoblasts isolated at 

 the 32- and 64-cell stages; and inasmuch as cells of the apical or- 

 gan derived from the i^-^ cells, and secondary trochoblasts derived 

 from the i^- cells, also differentiate typically when the isolated 

 micromere is allowed to segment continuously in the calcium- 

 free water, and the cells are separated more or less completely 

 after every division, the conclusion is unavoidable that these cells, 

 too, may undergo their characteristic development in complete 

 isolation from their fellows. Less detailed, but hardly less con- 

 vincing, is the evidence derived from the isolated ]4. basal, the 

 tV basal, or the isolated second quartet-cell ; and it can hardly 

 be doubted that the individual products of these respective cells 

 are, like those of the first quartet, definitely specified in greater 

 or less degree. 



The general conclusion thus reached in the case of Patella is 

 sustained by the development of larger masses of cells derived 

 from the earlier stages both of Patella and of Dentaliiim. The 



