Experimental Studies on Germinal Localization. 215 



two secondary ones, one-fourth of the apical organ and a group 

 of small ectoblast cells derived from i^-^, the whole structure 

 closing to form a morula or blastula-like structure, but otherwise 

 differentiating typically without gastrulating. In the aquarium 

 these larvas gradually disintegrate in the course of the second or 

 third day, the trochoblasts being always the longest-lived of the 

 cells, and often continuing to swim actively when the remainder 

 of the larvae has gone to pieces. I have not followed the details 

 of the development of the corresponding Isolated cells of Denta- 

 lium; but it is clear that their general development is closely simi- 

 lar. The one important difference, pointed out above, is that in 

 Dentaliiim only the micromere from the D-quadrant develops an 

 apical organ. As my experiments on Dentaliiim showed, the de- 

 velopment of the apical organ in this form is determined by ma- 

 terial that originally lies In the polar lobe, and no other conclu- 

 sion seems possible than that this material is in Dentaliiim finally 

 isolated In the posterior micromere (id), while in Patella the 

 corresponding stuff is equally distributed among the four mlcro- 

 meres. This is doubtless due to a different relation. In the two 

 cases, of the original segregation pattern to the first two cleavage 

 planes, and is perhaps connected with the absence of a polar lobe 

 In Patella. 



2. The primary trochoblasts (1-16, 1-32, i-64-embryos) . 



Exceedingly clear and interesting results are given by the Isola- 

 tion of the primary trochoblasts (i") or their products. If a 

 single trochoblast be isolated at the i6-cell stage It divides equally 

 twice in succession, but no further division takes place (Figs. 

 34-39) . From the eighth to the tenth hour each of the four cells 

 becomes ciliated and the group begins to swim. After twenty- 

 four hours the group Is swimming with great activity, and each 

 cell is found to bear a series of powerful cilia arranged in a 

 transverse row, exactly as In a normal embryo (Figs. 40-42). 

 The cells vary In arrangement, sometimes lying in a single plane, 

 sometimes having shifted so as to interlock in a rounded mass. 

 Exactly as in a normal embryo the action of the cilia is more or 



